Please help me. I'm all set to invade mars, but I don't know how to.

As above, if the planet is uncolonized a colony ship will do. You seem to already know that, but I've learned never to take things for granted.

If it belongs to someone else, you need to research Planetary Invasion. This is in the same tree as ship weapons and defenses, so you will need to research a few techs into that before ti branches. If you're playing Twilight, your tech tree may have different names for similar techs.

Once you have the planetary invasion tech, you need to build a transport. The premade Transport ship will work, but most people design their own ship to minimize costs - transports are like colony ships, they are one-use only. The preferred direction to transport design is to put only the transport module on the cheapest hull you can fit it on. Add engines only if necessary. Range is almost never necessary. Weapons are also optional and are rarely needed or wanted.

Transport vessels need not be armed--just clear out any enemy ships from orbit with your own regular combat ships before sending the transports in.