Look for the Beginner tutorials which detail everything for absolute beginners.
Recolouring/retexturing sim content like clothes, hair, skintones, eyes, etc.
Change body shapes and create new clothing meshes.
Default skins and eyes, townie-friendly stuff, etc.
Creation of the sims themselves, including celebrity/character sims.
Recolouring and retexturing objects.
Creating all new 3D object models and everything related to them.
Changing the way objects work.
Building the perfect lot or house - all the cheats, tricks, and tips.
Making the most of your gardens with planting and architectural elements.
Walls & Floors
Making walls, floors, and ground cover.
Careers & Majors
Creating new jobs and majors.
Hacks & Game Mods
Creating hacks and other game mods.
View a big list of everything.
Please start with the level you feel most comfortable with, or if you are a total beginner, see Sims 2 Modding Start Here