Well, there are tons of pages about procedural content around, first focus on one thing so you can understand how stuff is created, but basically the approach I am following is first add a lot of parts that can be mixed together in many different ways with different colors then create a random generator variable that uses a seed and that mixes all parts by using the rules you want, very simple at the end but allows you to create a large variety of assets. Problem is that most of the times without good rules you get crap
but sometimes, and I would say only sometimes it gives you a wow moment! I didn't think it will look that good! and you may even think that you may never had thought about that before, it just happens. So you spent a lot of times running against your rules to see what brings the best most of the times.
I have some examples on my indie db page, with character faces, floor textures, guns, now planets, and will add many more procedural things even dungeons.
For planet textures for example all is math based, some noise, some fractals, some random colors with a especific random seed for each planet , mix all those textures on a sphere and that gives you a planet. If you are more picky, you can even create humidity maps and rain distribution with noise so you can create forests and so on...
