Best thing you could do is hook up with someone that's a good designer geared towards web application design.
To be quite honest, all of us have our professional field of work. It's never the cake choice having a designer also work on the programming, or a programmer having to do the design as well.
I'll justify that by saying that the end result won't be the best one. If your a programmer that also has good experience as a designer, but your a programmer first, you'll notice that the design part of the work is always going to be the bit that's taking you more than you ever manage to calculate and also aggravate you in return. I've seen it happen, and it's really messy, when a programmer who can also be critical on design takes up to do the design part - invests a lot of time, never is really satisfied with what he does, and just gives up in the end and contracts someone to do it.
It's that little trick you always play on yourself. You instantly get a few ideas and think you can do it and that it'll look just great... but when you sit down it just never comes together.
If, by chance, you have no other solution than to also do the design, make sure you notify the client that you are not a professional designer, and that it'll take you times more to do the actual design than it'd take a professional designer and make the client know that he'll have to pay that extra time.