Go
here first. Basically there are two types of files you can download:
packages and
sims3packs. With sims3packs you download them and then install to the launcher (you can double-click, or there is a more complicated way that involves putting them in your "downloads" folder before you open the launcher). With packages you need to set up a mods folder. The "Installing Sims 3 Package Files" page I linked you to is really helpful; it walks you through step-by-step and gives you everything you need.