need some help creating a server

first, my english is not that well, so if i make an typo’s, i’m sorry.
second, i’m a complete idiot with computers,so please, be as specific as plossible!

now, the problem.

i’m trying to create a server were a friend and i can create a server, nothing special, for a few hours.
just that we can play on the first map in the list (don’t know what’s it’s called…)
be both have garry’s mod, and both have steam.
we also have 2 computers, wich are on the same network, and use the same router.
i’ve never done anything online, or with lan.

if someone needs some more information, please say.

i hope someone can help me!