"You wanna get high" - meaning yeah life is a bummer, but we can easily forget that.
"Don't forget to bring a towel" - It's like love itself, bring a towel and everyone will love you. See also Hitch Hikers Guide to the Galaxy.
"Oh man, I'm so high right now, I have no idea what is going on" - You're damn straight!
Towelie is also a rather heavy marijuana user, and makes no secret of it.
Most Memorable Moment:
Towelie: "Don't preach to me, fatso!"
Cartman: "I can preach to you all I want 'cause your stupid!"
Towelie: "Your stupid!"
Cartman: "Yeah, but your a towel!"
Towelie: "Your a towel!"