Use the keys:
W- to move forward
A- to move left
S- to move backwards
D- to move right

Thanks for reading my guide, I spent so much time on this, its highly appretiated!
~Lucífer

EDIT:
Use the key: V to enable noclip:
Then hold the 'spacebar' to move up.

Note: this only works if your are a moderator or admin.

RE EDIT:
If not in the previously mentioned 'no clip'
Press the keys:
Spacebar - to jump
Ctrl - to crouch

RE RE EDIT:
Hold the 'Shift' key to sprint.

Thanks again,
~Lucífer

RERERE: EDIT:
How to fix textures --> https://www.youtube.com/watch?v=GQwSCZuzbUY
I do not own this video, only linking it to help other people :P

REEEEE EDIT:
After buying the game navigate to Garry's mod then press THE BIG GREEN PLAY BUTTON.

Cheers,
Lucí

EDIT ON EDITS:
Press the key's:
'alt' and 'F4' simultaneously to close Garry's mod

A comment on my profile would be awesome!
Peaceout,
Lucí