For this type of testing you really want to be looking at some type of fuzz tester. SPIKE Proxy is one of a couple of fuzz testers for web apps. It is open source and written in Python. I believe there are a couple of videos from BlackHat or DefCON on using SPIKE out there somewhere, but I'm having difficulty locating them.
There are a couple of high end professional software packages that will do the web app testing and much more. One of the more popular tools would be CoreImpact
If you do plan on going through with the Pen Testing on your own I highly recommend you read through much of the OWASP Project's documentation. Specifically the OWASP Application Security Verification and Testing/Development guides. The mindset you need to thoroughly test your application is a little different than your normal development mindset (not that it SHOULD be different, but it usually is).