Sh4dowIn the midwest it's mostly volunteer (with the exception being Mount Bohemia), in the west they get paid. Probably in the east too.
You need OEC (Outdoor Emergency Care) or EMT training. Some places will only accept one or the other, some places will accept either one. You also need CPR training. OEC/EMT is a course that takes several weels (I think 6-8 weeks). CPR is a few hours or less just one day. Don't get OEC or EMT without looking into which one your local ski area prefers or requires.
Some ski areas take a while to start patrolling at. Snowbird will only hire someone as Ski Patrol if they work at Snowbird as Winter Trail Crew first. It's essentially the entry level Ski Patrol. Alta will only hire people who have patrolled somewhere else for ~10 years first. Other ski areas might have similar requirements. Some will hire you right away if you show them you'll be good enough. They might require you to ski with them the season before to show them that you're a good enough skier (that's how Arapahoe Basin does it).
Also, this is all specific to the US. Not sure about other countries, but I assume that at mountain areas they get paid. Not necessarily small ski hills (like the midwest).