The computer will not colonize planets with less than 15 quality, and for a good reason : morale will drop very low if you do. In the beginning of the game, shoot for 15+ planets, as quickly as possible. The AI knows where they are, so you must hurry to grab the one available. Do not hesitate to build a little fleet of colony ship ASAP, and buy colony ships of alien civ if you see them approaching a good planet.
However, if you spot some very influent planet, in the 13-14 range, it may be worth it, since Soil Improvement and Habitat Improvement will rise it to acceptable level. And maybe you'll be lucky with a planet improving event. No need to hurry anyway : the AI won't take them. By the time you get Terraforming, you can use a lot more viable planets than the AI, thus reversing the cultural influence on a sector by colonizing second hand planets.
As soon as possible, build the love mushrooms. It will help offset the initial pop advantage of the alien civs (in case you're not playing with version 3A), and you'll quickly lead in population.
Military fleets are costly, do not build more than you need to deter the opponents, unless you're aiming at a conquest victory, of course. The monthly cost would be used better elsewhere... When you feel the war is coming (hostile neighbour) put 100% of your money to military, you should be able to garrison your planet with that. When at peace, alternate 100% research and 100% social when you have enough improvements to build. I found it worked better than sharing, because the sharing is done at the empire level and not planet by planet.