Usually at small punk rock type clubs they'll have a rudimentary PA set-up for you to scream through... just loud enough to be audible but unintelligible when they're blasting half stacks (and sometimes full stacks!) in a cramped hall that has room for less than 200. A local two-bit garage band in a marginal/underground urban scene would typically play such joints. Otherwise, if they had a gig at a house party or something or were doing some outdoor type thing, they and the other bands (or whoever's throwing the party) would rent a PA system for the night.

If you're in your garage, rehearsing (this is again assuming you're a two-bit garage band)... ehhhhh... usually you just kinda holler real loud... though it'll be drowned out if you've got half-stack tube amps.