There are numerous solutions out there to your problem. A good one on that works on Android and Windows Mobile is http://www.gocanvas.com
If you are trying to use iPhone you can try http://www.iformbuilder.com/
Keep in mind, one of the downsides with using a web based solution, is that it won't work when you are offline (out of coverage) - which is needed alot with signature capture.
Good luck!