You'll definitely need to help us out to diagnose the problem. That's not nearly enough information.
Can you start by including some information on your system and what version of AE you're using?
Also, I think the best thing to do might be to attach some screenshots of both your timeline, effects palette, and composition window.
All that being said, I'd check your resolution settings (both the resolution of the comp and the preview resolution) as a starting point.
If you are new to AE please start here: Basic Workflow
Pick a video file in the Project Panel and drag it to the New Composition icon or select File>New Composition from Selected.
Make sure you are using a default workspace, make sure that the Composition Panel zoom factor is set to 100% or 50% (first box at the bottom of the comp panel) and the Comp Panel resolution is set to Auto. Make sure that fast previews is set to Off (Final Quality) or Fast Draft. You should use Adaptive Resolution only if you understand how that works.
If your video does not look normal after following these steps then drag a full resolution screenshot into the Reply field on this forum and let us see what's going on. To give us a useful screenshot you should have the footage that is giving you problems selected in the Project Panel so we can see the footage details and you should select the footage in the timeline and press the U key twice to reveal any modified properties in the layer. For example, This footage is out of position and scaled up.
If you created your own composition then include the comp info or a screenshot of the composition settings (Ctrl/Cmne + k with the comp selected) so we can see if the composition settings match the footage specifications.