Need help for first time modding

Hi, I'm planning to make make my first mod, but any type of mod, a completely new gun mod. I'm thinking about making the WA2000 for the L96, but I'm not sure what programs to use, and how to do it, so I am asking if you guys can help guide me into the right direction of programs to model and make my gun. I have Photoshop CS4, and I'm need to know how to make the gun is .rez format, cause my computer won't read it in any other. Thanks ahead of time.