It depends on what you want to make. If you want to make clothing, you need The Sims 2 Bodyshop (that comes with the game) and an photo-editing program (like Adobe Photoshop). If you want to make a recolor of an object you need the photo-editor as well as SimPe. If you want to make a mod, you just need SimPe. If you want to make a completely new object you need a program that can make 3D meshes. Some of these things are more complicated than others (the stuff I listed first is easiest and the stuff I listed last is harder). I suggest that you go to www.modthesims2.com
- they have tutorials and forums where you can ask other creaters more complicated questions.