I often cannot differentiate between what I'm saying is slang or proper English.

Boak is word that essentially means 'barf'. I've never heard anyone say barf here, ever.

There's too many to list, honestly. But Boak is a favourite word :).

Apparently "hella" is not universal, but growing up in the Bay Area and California had me thinking otherwise. That's hella weird. We say "hella" hella out here.

We're hella cool. >.>

I guess they call it an amplifier word. Otherwise I'd just say it's a more hip way of saying "very".

i've been saying a lot of "mm'kay" in the style of Mr. Garrison lately

I've heard a few people say hella, but I think it was my friend who goes to uni in America..

We say "hella" here, too.

Some other slang that may be unfamiliar that's used:

Nans = Shenanigans. Or, saying you don't believe a statement, thus calling shenanigans.
Example:
Person One - I hear that Mildred Dipplethorpe's baby daddy is Thom Yorke.
Me - NANS!

Firemost: The greatest. The best. Fire is good, and this thing is the most fire. It is super awesome.
Example: Dude, I just ate the firemost brownies this morning!

Mad: Well, this is just used as in crazy. Usually in reference to drug activity, although occasionally used to reference other things as the user sees fit.
Example: I'm sorry I couldn't make the party, man. I was mad tired.

Fo shiz: For real / seriously. Could be a question, or could just mean I'm serious.
Example:
Person 1 - I crashed my car into the college today.
Person 2 - Fo shiz?!
Person 1 - Fo shiz.

Awww, sheee...: Aw, ****. General statement of concern and / or disbelief.
Example:
Person 1 - They're ticketing everyone parked in the side lot, so I hope you're not parked there.
Person 2 - Awww, sheee...

Precursor to rage meme FFFFFFUUUUUUUU!!!!!!

I invented a phrase a while back that I'm trying to spread, but I don't think it's working: "Pop this coolie" instead of "about to leave". So, "Hey dudes, I'm about to leave" would be, "Hey dudes, I'm about to pop this coolie". It just popped into my head one day and I was amused.

I've decided to incorporate one of these into my vocabulary each day. :D

Some nans going on er'!

Alot of the time we say "I'm absolutely out my 'tattie'" being that you're really drunk or when the drugs hit you.

Sapnin: it's basically 'what's happening?' shortened to 'wits hapnin' then sapnin. If I was asking what someone was doing I'd say sapnin.

Goon: if I call you a goon, I'm essentially calling you retarded.

Aww wit: difficult to explain so I'll put it in context.
P1: I just heard boys noize pulled out of playing at Rockness.
P2: awww wiiiit, thats ****e.

Does anyone else use shitfaced when describing drunkeness?