You can not take a Class 0 planet and make it useable. But you can "improve" colinizeable worlds with techs from the "planitary improvments" branch of the tech tree.
If you are only getting two planets you will have to do more of a push to colonize in the early game, it might help you if you start you starport building a colony ship in the first turn and defisit spend with high military while you can, and send the colony ships out early. You will get more planets this way.
But really it all comes down to tactics. The more you play the more you will no when to spend on what.
PS. Check out the "colonizing worlds" totorial, it might help out.

When you start a new game make sure that the habitable planets setting is on common or even abundant. If you want a tun of good worlds set all the settings to abundent. However the will mean that your enimies will also end up with a tun of worlds, and change the game play alot. As you get more advanced you will find the type of galaxy you like to play in.

Thanks, have fun.

Yea, I know. Habitable planets aren't distrubuted very evenly. There will be systems with all Class 0 (Undead worls) while others may hyave 3 planets or more that are very good planets class 9 or higher. I've even found a star system that was bright yellow with all planets above class 10. So it's all random, but I think it could still have been more distrubuted a bit more. Yes both my setting where on Abundant, both number of planets, and habitable planets in a huge galaxy.