To thoroughly enjoy Garry's Mod, one can start by immediately heading over to the Options. The reason is that Garry's Mod does detect proper options, but does not compensate for lag-spikes. To avoid this, simply set two or more recommended settings to one setting lower than the suggested, as shown by *.

Another way to enjoy Garry's Mod is to decided how and what you are going to play. There are plenty of gamemodes to choose from including but not limited to the following:

Trouble in Terrorist Town or TTT (A popular detective gamemode)
Murder (A game of trust)
Stalker (Just like the Hidden, but harder)
Prop Hunt (Kinda like hide and seek, but with props)
Hide and Seek (Really hide and seek.)
Traditional Sandbox (Building)
Basic Team Deathmatch (A generic take on modern first-person-shooters)
Pirate Ship Wars (In this gamemode you must sink the other team's pirate ship before they sink yours)
Garry's Mod Stranded (In this gamemode, you play as a survivor, stranded in a foreign environment, and you have to survive. Just be sure you don't run out of Health, Hunger, Thirst, and Fatigue.)
Garry's Mod Racer (Recommended for use with the "SCARS" Addons)
MORBUS (Similar to TTT. In this gamemode you have to survive in a science fiction environment, while being hunted by Aliens)
Elevator: Source (a single or co-op elevator experience that is different each time you play. What floor will you stop on next?)
Roleplay ("Rp" or ''Darkrp'') (More or less a simulation of videogame life)
Flood (In this gamemode, you build boats to escape the oncoming flood. Once you've done so, you must then destroy other players boats to earn money for more props and weapons the next round)
Jailbreak (As a Prisoner, you can choose to follow the orders from the Warden, or go against him and try to kill the guards. As a Guard, you keep the peace)
Gmod Tower (GMTower is the ultimate social experience to play with your friends in Garry's Mod, it has a lot of mini games and it's really fun)
Deathrun (Try to run and get to the end of the course, while dodging the traps along the path. As a Runner, you have to run to the end of the Deathrun course, while dodging every obstacle along the way. As Death, your job is to try and kill of every Runner possible using every contraption on the Deathrun Course.)
Hunger Games (In this gamemode, you play as a Tribute in the Hunger Games! Play with your friends, or trust no one and kill everyone on sight!)
Minigames (Exactly what you think. In this gamemode, you and many other people chose between various minigames to compete in. These range from Climb, to AWP, to Surf, to Obstacle Course, and more.)

Don't be a douche in online gaming.

Do not RDM or RDV on TTT, they sure hate that and you'll be punished. (Random deathmatch or Random votekick)

Another way to enjoy the game, is simply to look up a "Garry's Mod Let's Play" on YouTube. These can trigger ideas and ellaborate on how Garry's Mod can become fun in various gamemodes.

Also, make sure you check out the workshop.

Have Fun.