I would offer a few thoughts.

First, GalCiv2 is really three games, not one, as they were developed/sold sequentially.  That is, Dread Lords then Dark Avatar, then Twilight of the Arnon, or DL, DA, and TA.  DL is a simpler game than DA and TA is more complicated than DA, but what one learns in each game can be applied to the more complicated successor.  You could always choose to play first the "sandbox" version of a game before TA, as those are simpler.

Second, I agree with erischild that the choices a player makes reflect the game style the player wishes to use.

Third, the various preset AI races are "tuned" to different approaches.  That is, one race may be war focused and another might be trade focused.  Thus, when playing those races, one might wish to play to those race-specific strengths.

Fourth, there are certain fundamental tech choices that should probably always be made early.  For example, some techs give bonuses, so research them early.  Look at Xeno Economics and Planetary Developments for two such.

Fifth, if one specific sequence of choices was optimum, there would be little use in other choices and the game would be shallow.  One of the criticisms of some early RTS games was that the Tank Rush or the Zerg Rush had to be revised away or else each particular game had no replay value.  Role playing games also work hard to make the various classes worthy, or else no one would want to play anything other than the mage or Elven warrior or whatever the uber character might be for that game.  An earlier 4X game now regarded as a classic, MOO2, had to be patched to prevent no weapons being used other than plasma cannons and an "initiative" feature added to prevent the attacker from always winning.

My recommendation would be similar to erischild's.  Choose a race, Terrans works, and play on some setting below "Tough" with a medium galaxy, two or three AIs, and maybe 5 minor races.

Oh, the one race choice I would suggest you maximize would be Economics.  It is not absolutely critical but it will make gameplay a bit easier with one's income higher. 

Yes, but you have to do it manually. For DL, use the GalCiv2.exe in the main folder of the game. For DA, use the GC2DarkAvatar.exe in the DarkAvatar sub-folder.

I have GC 2 Ultimate and when you click on the desktop icon the launch panel appears you can play that game (they are different designs for DL,DA,ToA, use switch chapter to change between them and then click play game is how I do it

The esayiest race to learn is Terran Alliance (how I learnt it) keep the default settings until your comfortable with altering them.I now create my own races and ship styles but it took me awhile and although I've been playing on and off for over 4 years I have by no means mastered it.

As others have suggested turn the game difficulty down to cakewalk or simple as on higher levels when I first played I was just starting out and my opponents would turn up with large hulled battleships and blow me to pieces (still do on occasion!)  As to what path to win thats down to you as well ( military/economy/influence/ascension/research ) experiment with it. For starting out the federalist party is a good choice.

My starting build on planets is 1 factory,starport,res lab,market centre,and entertainment if morale is low as for techs I research ion drive then communicator,xeno research,planetary improvements,xeno economics and go from there. some  do all factory/research planets but for starting out I suggest a balanced approach until you get the feel of the game and what you want to do with it.

Personally I love the ship builder on it and have made so many ship templates i've got them coming out my ears

If you havent already have a go at the dread lord campaign to get a feel of some of the basics or practice in the sandbox games thats what I do.hope this helps

An important tip that has, so far, been missing is howto grow your population.

During the early game, try to run your taxes so low that the population on your homeworld has 100% approval. It will grow much faster that way than when you have it at 99%. Once you go deep into the red, it's time to start taxing the suckers...

Second tip: in the same screen where you adjust taxes, also ramp up the production slider to 100%. It's better to go bust quickly, rather than build everything slowly.

As for the rest, experiment and don't be afraid to try things out. Although some techs are better than others, most have at least some value andcontext is always important in determining that value. The AI at Tough doesn't cheat, but it probably knows the game a lot better than you, so keep your playing level below that until you have a feeling you understand the game better than the AI, then ramp it up till it starts to hurt again.

For ToA it is more important that you know what you're doing, as the techtrees have gotten more complicated. I find DA better than DL, but about as easy to understand. Playing with the Terrans gives the most neutral experience as they can play any tacticwell and theirawesome diplomacy really buffers you against some of the nastier points of the game. Good luck!

Plenty of great bits of advise from all contributors here, just thought I would add a few more words to help encourage your 'fun' from the game. I had quite a diffiucult time getting involved in GalCiv II during my initial playthroughs. I blame a vast majority of this on the fact the I am not the greatest fan of the sci-fi theme, but it luckily seems you do not have this issue. The first thing that helped me to really develop an appreciation for this game was seeing just how in-depth each new species was. Each species has been fleshed out with mini histories, rivalries, tendencies, etc.. Even beyond that, each species has its own research trees, buildings, and units that are (generally) unique to them. While all this can be a little daunting at first, it is amazing how well it all plays out in a grand 'space opera' when you let the atmosphere overtake you. I would argue that this is what GalCiv II does best. Atmosphere and personality, all with options that add to those qualities. Now, on to your specific points....

• Most have agreed that the best race in order to get a feel for the game is the humans...Not the most exciting, I know, but they really are amazing for testing out ideas and interacting with the other species...learning just how different each one is. This is mainly because of 2 advantages the humans have.

1- Expert Diplomats- This ability really helps to ease you into the game as it drastically affects how likely the AI is to go to war with you. Thus you can spend more time making mistakes and testing the game without dealing with constant war because you appear weak. This ability also encourages diplomacy, a very important (if a tad bland) part of the game where you can learn of other species' technologies, the values the AI places on those techs, etc.

2- Simple Tech Tree- The humans have the simplest of all the tech trees in the game. The tech trees are by far the most overwhelming part of the game for most players and while the human tech tree is by no means boring or basic, it is the most easily understandable tech tree. After playing around with this tree and getting a basic understanding of it, it is much simpler to understand the nuances that the alien species add to the game.

If these two options do not appeal to you...forget it! Play with whatever race you like the most or that fits your playstyle. I would stress that the most important thing is that you play a few games with the same race in order to get a feel for what is 'normal' and what is special to another species.

• Attributes are best chosen to complement your strengths. You can have personal choices all you want (i regularly do) but the best strategies revolve around playing to your strengths in this area. Making up for weaknesses is fine as long as it is a small weakness. If you have -10 military (see humans) then it is cheap and easy to fix that... just try not to battle every weakness that some of the other races have or you can get strung out. The basic idea behind attributes is they are a way for you to 'flesh out' your races' personality and play style the way you would like. **These suggestions also work for social parties.**
• Research is another one of those 'as you like' areas as well. Again, sticking to your strengths tends to work better, but cherry picking is not going to throw you drastically behind with no hope. I reccommend that you select a few that fit your playstyle more and dedicate to those, while cherry picking when you feel lacking in areas. 'Bee-lining' or dedicating to one tree works well but you really need to know what you are aiming for and what your plan is with that, as well as how to make it most useful since u decided to get it so early. In other words...a much better strategy after you know the game and a few of the tech tree surprises.

I would like to add for the techs that it is going to be overwhelming and that you just need a few playthroughs to see what awaits you and where problems arise. GalCiv II is however great at still keeping you in the game even if you have made a few mistakes along the path.

• As for buildings...Yeah, trickiest one for me to give advise on here. My best words of wisdom are that you should only build a building when you have a clear reason for it. "This factory would be good here because I will be building a lot more buildings and ships here" is great..."Oh, a factory could work here" is not going to cut it. Another consideration is that planets are usually at their best when they have a dedicated purpose. Planet X buils ships, Planet Y builds science, Planet Z has trade routes...etc.

Everyone above me has posted great tips, and I would emphasize that it is essential to get the production slider at 100% and approval to 100% for a lot of the early game. Finally, take all this only as much as its worth. I am not an amazing player but I do regularly keep up with the big boys. I have quite a few games under my belt, but most importantly, I am just someone who loves GalCiv II.

-Scanian