Why can't I do push-ups?

What type of weight training are you doing?
I say that maybe you have not been developing your upper body enough? I used to be the same but really heavy weight training has allowed me to be able to do 10 reps at first attempt.

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.

Best Wishes,

Rosa