You can scale to fit horizontally by selecting the layer in the timeline and adjust the scale property or use the keyboard shortcut Shift + Ctrl/Cmnd + Alt/Option + H.
If the video is larger than the composition you should be good to go but you may be able to improve the look of the scaled down video using unsharp mask. If the video is significantly smaller than the composition you might want to use a 3rd party plug-in like Red Giant Instant HD.
After Effects CC introduced Detail Preserving Upscale which also helps with the upscaling problems.
thanks a lot for providing a solution for my problem sir