fit and fab,
It seems like you have not been taught how to do a push up correctly. Sometimes, due to our level of physical fitness you need to start out doing push ups on the wall (yes, on the wall). Here's a good visual of what you should be doing: http://www.sparkpeople.com/resource/exercises.asp?exercise=107
Once you can master those with ease, then you can move into a modified push up, or what some people refer to as a "girl pushup"and that looks a little something like this: http://www.mayoclinic.com/health/modified-push-up/MM00735
Then once you have mastered modified push-ups, you can start doing regular push-ups. But, like everything else, you have to walk before you run. We all have muscles in our arms but it doesn't quite mean that those muscles are ready to jump into a push-up.
Once you feel comfortable enough doing modified push-ups, you check this site out to give those regular push-ups a try. http://www.ehow.com/how_3190_proper-push-up.html Don't be too hard on yourself, regular push-ups are a challenge on everyone. One thing should always remember is "keep your butt down". When you feel like its coming up, keep it down. Your body should always be straight during a push-up.