I have a peavey 212 combo amp.. but if im going to play shows with my band im gonna need more volume with that. its a $900 dollar amp so it doesn't suck, so i wanted to just add a cab to it but im not sure whether i can or should do that. im not necessarily an expert about amps so thats why i'm asking what i should do if i wanted to play shows. thank you
Stick a mic in front of it. Problem solved. BIG AMPS aren't always the solution. If they were, why would anyone sell high end combos?

You can definitely add a cab, but I wouldn't bother.

Or, get a Marshall half-stack. They are pretty awesome too, and what's more they LOOK the part.
Marshall amplifiers are the truest purveyors of rock and roll known to man.

"And give a man an amplifier and a synthesizer, and he doesn't become whoever, you know. He doesn't become us."

Holy crap, check this out!
How many watts is it? A 2x12 pushes a lot of air ... I can play over a drummer without any problem with my 15 watt Tiny Terror and a 2x12. Before that I had a 50 watt Mesa/Boogie with a 1x12, and that was even louder. Like, never went past 4 1/2 louder.

Stacks aren't really necessary, unless you're playing a HUGE venue, or just want to look cool. And if you just want to look cool that's okay - stage presence is a big part of any live show. Just don't think that you NEED one. Actually in most bars and small clubs, you'll probably make people think you're a tool it you bring a half-stack.

As far as extension cabs go, check the back of the amp to make sure it's wired for an extension cab. It probably is - most high-end, non-tweed amps are. Then just find another 2x12 cabinet (or a 4x12 if the extension jack overrides the main speaker) with good speakers and you're ready to rock. Personally I prefer Celestion Vintage 30s because they're extremely loud and sound good with any type of music, but anything with a name brand on it should be decent. And really, if you're playing bars, I guarantee that your audience won't hear a difference live, unless your speakers are awful.
You won't need anything else. a 2x12 combo is plenty sufficiant.

If I were you, I would worry about having backups for EVERYTHING. Nothing is like having a gig ruined because a piece of gear craps out/gets ruined. Just make sure that if any one piece of essential gear breaks down, you have something to take it's place in an emergency.
Gibson SG Standard
Gibson Les Paul Traditional
Cort Explorer
Squire Standard Strat rebuilt with Fender USA parts
Squire Tele
Krank 1980
Orange Tiny Terror
Traynor YCV 50 Blue
Peavey Vypyr 75

Will fly for food. Call me Dylan
I am sure you have plenty of volume. If I where you, I would spend my hard earned cash on a compressor, delay, EQ, or new pickups before an extension cabinet.

If you really want a cab though, a 2x12 should do ya fine. (I personally don't even really like 4x12, gets soooo boomy sometimes)
"If you're looking for me,
you better check under the sea,
because that's where you'll find me..."