how do i attach a gun to my character

My opinion:

Download Blender (Free, and works). Or if you have money, or are a student and have a .edu email, you can get any Autodesk program for free. Get one.

Now, model some hands and a gun.

Now make that a child of the FPS Controller.

What you can do, but is very pointless/worthless:

Download someone else's gun and hands/arms.

And do the same as above.

Now, as for shooting, you'll have to learn scripting. So for that. www.google.com, and start learning Javascript or C#.

Good luck!

Start reading here: http://unity3d.com/support/resources/tutorials/fps-tutorial-part1

I think you will find your gun there as well.