1) Install the steam application for mac or windows

2) Create an account for steam then sign into the application

3) Go to the wallet and funding section then add $20 to your account

4) Go to the store and type in "Garry's Mod" in the search bar

5) Go to the game in the store and click continue to check out

6) Purchase the game for yourself instead of gifting it to the friends you may or may not have

7) Go to your library home section of the steam application and install Garry's Mod

8) When the download is complete press play.

P.S Try not to crash :)

EDIT 1: Watch out for chickens

EDIT 2: Watch out for friends that are good at pickling cucumbers
RE:EDIT (5): Avoid all friends unless they have no intentions of sexual items or intercourse.

EDIT 3: If you accidentally install source filmmaker, stray away from the NSFW items and restart from Step 1

EDIT 4: If you fail and find a chicken and enjoy eating it; start from step 4 and continue attempting to again watch out for chickens.

EDIT 5: If you get slime rancher by accident or on purpose, enjoy that and when you finish the game start from step 4.

EDIT 6: Stay away from kitchen technology, unless any part of your PC is stored in it.

EDIT 7: Avoid morphing animals