Internet acronyms:

LOL = Laugh Out Loud
LMAO = Laughing My A** Off
ROFL = Rolling On the Floor Laughing
WTF = What The F***
BTW = By The Way
IMO = In My Opinion
IMHO = In My Humble Opinion
AFAIK = As Far As I Know
BRB = Be Right Back
BBS = Be Back Soon
JK = JoKing (or Just Kidding)
BF = BoyFriend
GF = GirlFriend
OMG = Oh My God
BS = BullS***
AFK = Away From Keyboard
FYI = For Your Information
cya = see you (C = see, ya = you)
TTYL = Talk To You Later
WB = Welcome Back

I bolded the most common ones.