Depends on your department. They might be giving you other things--I know of some that offer tuition reimbursement, exemption from certain taxes, and even health insurance--that make applications competitive. They might also be large enough to always have a full crew available, which makes fitting new people into the schedule inconvenient. Most aren't like that, though, so it's pretty easy to get in. Drug use and previous felonies are usually disqualifiers, though.
I'd recommend knowing your EMT stuff, and having a positive attitude about community service and all. If they offer training, I'd also be clear that I'll commit to giving them service in return.