Well you have taken the first step - you are aware of what you are doing, you may not believe it but most people do not realise that they foot fault.
There are various reasons why you may jump or hop on a serve - bear in mind you may have more than one of the below. For each I have listed some exercises that may help:
You are trying to hit the ball too early
If you hit the ball as it's rising (or out of your reach)- you are effectively 'chasing' the ball away from you, if you are already at or near your full extension the only way for your body to go is...up, resulting almost certainly in a hop.
Try to keep your elbow relaxed slightly bent when you go to hit the ball, then you will subconsciously wait until the ball is in reach (i.e. you're more than likely to chase the ball with your arm rather than your whole body).
You are throwing the ball too far in front of you - or not in the right place
If the ball is too far away when you go to hit it or you are not sure where it is going to go, the natural process of the body will be to reach and ultimately hop for the ball. The only real solution is practise.
Line up as though you are about to serve, throw the ball as normal but do not hit it. Repeat this 10 times - each time noting where the ball lands. If the majority land more than 18 inches from where you are standing or land in wildly different place, then the chances are you are not throwing up the ball as well as you might.
When you are playing a match - remember you can let the ball bounce if you haven't thrown the ball up to your liking!
You are too keen to get to the net
It is drilled into all of us the importance of getting to that net. So what can happen is that before the serve is completed many people are already trying to get to the net.
You have got to have faith that you can get to that net - there are two ways...slow the serve down or get yourself faster !!
Ways to improve your speed into the nets are - keep on those toes, take slightly larger steps after the serve or get faster through speed practises like shuttle runs etc.
You could also practise your mid court volleying and half-volleying skills so you don't feel the need!
You are knowingly cheating!
Stop it - it is a game that should be played for fun - remember persistent foot faulters that have been told many times, are seen in the same light as people who deliberately try to cheat the score or call balls out when they're in.