"Towelie, you're the worst character ever" - You're so not! :)
"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!