What is studied at school is normally formal english, because gives you the base, and this is right and good. But the language that is spoken between people, the youngs especially, is a little bit different. It’s even more used on internet or in the lyrics of the songs (try to go on a Youtube video and read the comments ;) ) Personally I often get confused when I try to understand some of the thousands of expressions and sentences used in the unformal english but the trick is just try to guess (like "horse sense" that means to do or decide something with carefulness and wisdom). One of the features of this kind of english is the presence of a lot of abbreviations. Here there are some common ones :
Gotta - from “got to” “have to”. Example: I gotta go
Gonna – from “going to” , something that is close to happens. Example: Today is gonna rain.
Wanna – from “want to”. Example: I wanna play basketball.
Kinda – from “kind of”. Normally is used also to say something between yes and no, or a little. Example: I’m kinda in love for a girl ; I’m kinda tired.
Watcha – from “what are/have you?”. Example: Watcha lookin for?
Cuz – from “because”
(This kind of unions of words can be made with many more, like “musta - must have”, “coulda”,”woulda”,”shoulda”,”hadda”,”mighta”,”oughta”, "dunno" and so on....)
Wazzap,wassup, 'ssup – from “what’s up?” that is a typical expression used between close friends, and it’s something between “how is going life?” “how are you?” and “news?” (The closest expression here is “gimana?”)
Gotcha - from “got you!” (Literaly means something like “dapat kamu”) It’s used for example when you make a joke to someone and he “fell” into your joke. It’s also used when you maybe understand from a friend of you something that he want to keep secret.
I have a crush on you - This in not a abbreaviations but an expression, and it means something between be interested and a little bit in love for someone.
Moreover there is another category of unformal language that started to exist with the boom of the communications technology: the acronim. They are something not really used in speaking but really present on internet and used a lot in the sms. Here there are some
LOL – from “lot of laughts”. In Bahasa Indonesia “Tertawa ngakak”. Is probably the most famous one and is used not really when something makes you laugh a lot but just for something that makes you really smile.
ROTFL – from “rolling on the floor laughing”. “Berguling-guling diatas lantai sambil tertawa” in Bahasa Indonesia. This is an expressions used more when you read something maybe really stupid but you cannot stop laughing.
OMG – from “oh my God!”. Literaly “Oh Tuhan!”. Is an expression of amaze, used when you are surprised or wondered for something. Can be used for bad happening or good happening.
BTW – from “by the way”. “Omong-omong”
ASAP - from "as soon as possible"
GG – from “good game”. “Permainan bagus”. This is actually an expression that comes from the videogame, gamer of videogames, used to congratulate with the others players at the end of a game, but became usual to use it also for other situations like to praise a friend that reach a “good goal” or if you just enjoyed some news from a friend of you.
At least they exist also other kind of abbreviation like "plz" for "please" or "thx,tnx - thanks" "u r - you are" but this is something used also in Bahasa Indonesia.