how do (write) I own a gun?

sorry for my bad english but how do I own a gun for gmod.
I mean, how you write a simple gun?

The best way to start is by using a template or a SWEP making program. Both usually have bad outcomes, but its an easy start.

help my please, how i do it?

Download any SWEP Template from www.garrysmod.org.
I recommend this one http://www.garrysmod.org/downloads/?a=view&id=24031.
And follow the instructions.