There are loads you will come across especially in very informal and casual internet chat... I will list some of the more common ones you will come across...

lol - laughing out loud
omg - oh my god
atm - at the moment
brb - be right back
fml - fuck my life
g2g - got to go
nvm - nevermind
fe - fair enough
fp - fair play
wtf - what the fuck
tbh - to be honest
cba - can't be arsed
lmao - laughing my ass off
pmsl - pissing myself laughing

ROFL - rolling on floor laughing

SMTH - Something

IDK - I don't know

IKR - I know right

FUUUUUUU - Fuuuuck OR fuck youuuuuuu

GG - good game

TTYL - talk to you later

SUP - What is up?

BF - Boyfriend

GF - Girlfriend

ASAP - As soon as possible

BTW - by the way

GB - Goodbye

FB - Facebook

YT - Youtube

JK - Just kidding

JW - Just wondering

PLZ - please (May also see pl0x)

THX - Thanks

U2 - You too

UR - You're

AFK - Away from Keyboard

I'm afraid that I can't understand them except LOL.

I guess ATM is at the moment, right?

LOL

ROLF

LMAO

SMTH

IDK

IKR

FUUUUUUU

GG

HF
GL
TTYL
SUP
<3
BF
GF
ASAP
BRB
BTW
GB
GN
GM
FB
YT
NVM
JK
JW
PLZ
THX
F2F
U2 
UR
F2F
FYI
AFK

with this you might survive, I don't remember more atm :P ATM!