Here are some common acronyms that you'll often see when chatting to English speakers in an informal environment.
LOL - (I am) laughing out loud.
ROFL - (I am) rolling on the floor laughing.
ASL? - (What is your) age/sex/location?
BRB - (I will) be right back.
GTG - (I have) got to go.
BB - Blackberry
WTF - What the funk!
TTYL - Talk to you later.
OMG - Oh my goodness/god!
GG - Good game.
TY - Thank you.
ETA - Estimated time of arrival
WB - Welcome back.
ASAP - As soon as possible.