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 used to have a bunch of custom clothes that I made for my own game, and they were good! However, when I uninstalled the game long ago, it was all erased. Now that I've re-installed it and gotten back into the Sims, I'm gona be making tons more custom content :D