Never made any custom content before? Start with the easiest, most basic stuff (which you can make with all free programs) - Sims 2: Walls, Floors, and Ground Cover, Sims 3: Custom Patterns
Just installed the Sims 3 patch or about to? Read this article for helpful advice about patching and keeping your custom content working!
I'm a long time Sim player and the Sims 2 is perhaps one of my most favorite of all time games. No matter how long I've been playing it, I always go back to it.