New into AE notice that all the footage I am importing was shot native in 16:9 but imports into AE as 4:3. Poked around the preferences but I am not familiar enough with the interface to trouble shoot the problem? Is it comp settings er what?
If it's SD 16:9 Michael's suggestion is good, but you also have to check that your footage is interpreted correctly (right click on it in the project window) and that your comp is widescreen par.
If your referring to other kind of footage - 720p or i or 1080p or i, then post more details about your project.