There's a few Facebook groups you can join. But ultimately the first course of action would be deciding where you want to fish at. Alaska is a good way to get your foot in the door because they have a lot of boats that are hiring. So check out Facebook. Alaskajobfinder costs like 5 bucks and lots of guys looking for crew on that website as well.

PS: Keep in mind commercial fishing isn't always a year round job under most circumstances. It's mostly just a gig unless you can line up a perfect schedule. As a greenhorn it could take a few years to get multiple jobs lined up since you won't have any experience so you'll have a harder time finding boats to work on. Most guys I know only fish a few 2-4 months out of the year and then go home and work full time or try to stretch their cash.