I wanted to learn this for years but only today after about 2 hours of trying to find an EASY way to colorkey pictures i finally found a good website explaining it in a way even noobs understand. And in this case, i am a noob. All you need is the wonderful program called "GIMP" (Free Download, just google it. It's similiar to photoshop. Now that I used to work with photoshop i must say gimp has got three advantages: its easier AND its free AND you can do the same things as with photoshop).
Here's the link for my German friends who have the same problem with colorkeying
This is my first ever colorkey picture, it's probably not the best but it's a start: