Not sure what's still said in the common parlance, particularly since I haven't lived in the US since 2004. With that in mind, here is a little glossary in case my characters say something that may make you scratch your head.

'90s Slang

Aiight: All right, but cooler

Alrighty then: a sarcastic reply

As if!: another sarcastic retort (short for "as if that would really happen")

Booyah!: an expression of enthusiasm, pleasure, or approval

coo: cool (no one liked to enunciate in the '90s)

(to call) dibs: to declare the right to have something before someone else  

dude: a casual way to refer to someone (at first, as an alternative to 'man'; later gender-neutral)

hella + adj: really + adj. (e.g. hella cool)  

man: an alternative to dude, usually only for guys

NOT!: a way to reverse everything you just said; sarcastic  

peeps: people

phat: awesome times 10 to the 10 billionth power  

Sup?: What's up?  

sweet: alternative for awesome or cool  

That's wacked: That's so weird/messed up.

trippin': freaking out; acting weird  

Whatever: said in a sarcastic tone; to indicate one doesn't wish to speak any longer

Whazzup/Whassup?: originally in a Budweiser Super Bowl commercial. It caught on. 'Nuff said.

wicked + adj: (New England slang) totally/really/very + adj. (e.g. That's wicked awesome!)

You go, girl!: I'm so proud of you!

Hmm, it wasn't a very subtle era, was it? :D

Textspeak (Acronyms and Abbreviations)

AFK: away from keyboard  

A/S/L: age, sex, location (often as a greeting)

ATM: at the moment

BBL: be back later

BRB: be right back

BTW: by the way

G2G: got to go

IDK: I don't know.

IRL: in real life (as opposed to one's online persona)

jk: Just kidding

l8r: later

LMAO: laughing my ass off

LOL: laughing out loud

OMG: Oh, my God!

OIC: Oh, I see.

OXOX: hugs and kisses

pls/plz: please

ppl: people

qt: cutie

ROFL: rolling on the floor laughing

STFU: shut the **** up

TC: take care

TTYL: talk to you later

WTF: what the ****?

ZZZ: bored or sleeping

Outdated Terms and Concepts

AltaVista: one of the first search engines before Google became prevalent

AOL: America Online (back in the late '90s, it was the first internet provider to offer users unlimited access to the internet for a flat rate instead of paying by the hour or minute)

AOL Instant Messenger (AIM): one of the first texting apps (before that was a thing) that allowed you to communicate with people online; it wasn't mobile, however

blocking someone: the equivalent of 'muting' someone nowadays; blocked users were unable to send messages to the recipient who had blocked them

(dial-up) modem: a way of connecting to the internet that dinosaurs like me had to use before there was broadband; you connected to an internet service provider like AOL via a landline

instant messaging: the equivalent of sending someone a PM/DM nowadays in social networks; only the recipient can see what you're writing; later allowed peer-to-peer file transfer

LiveJournal: one of the first websites in the late '90s that allowed you to create blogs, before Facebook, Twitter, and other social media networks were invented