The official "The Sims 3" website has a section called The Exchange where players download sims, houses and more. The Sims Resource is a website where players can download custom content from. Another option is My Sims 3 Blog, which provides links to independent blogs with custom content.
After locating custom content that you would like in your game, you need to install it. Custom content for "The Sims 3" comes in three file types: .sims3pack, .package, and .sim. To determine the file type, look at the extension after the file name. Depending on the type, installation requirements differ. For example, a .sims3pack can be installed by simply clicking on the file, whereas a .sim file requires you to drag it to a specific folder on your hard drive.
Another fun way to get custom content is to make it yourself. The Sims Resource and My Sims 3 Blog offer excellent tutorials to help you get started.
Since custom content is user-made, you run the risk of corrupting your game file when you install it. To prevent this, back up your save folder prior to installing anything new. If you feel like your game is acting abnormally, remove the new content and revert to an old save.