I was a Warehouse Associate for 10 years. What you do really depends on the warehouse. I worked in a pharmaceutical warehouse, mainly I filled orders but I did just about anything else that was needed.
I could sort orders and check them of mistakes. Stock/Restock the shelves. Fill in for the shipping clerk. Take inventory, build shelves, sweep the floor, clean totes, move product. Even filing papers. Mainly just being crossed trained to fill in for people.
As for pay, it's not bad but that depends on the company as well. It's not a high paying job by any means but it's way better then retail or most fast food places.
It's not a rewarding job, you are basically just a cog in the machine. You won't feel fulfilled at the end of your day, just tired. It's also, in my case, quite a lot of walking, miles a day. Which will keep about 15 pounds off your waist.
It's also very fast paced, some warehouses have quotas that you have to meet, ours didn't, we worked until everything was done, sometimes 13 hour days.
How to get into that line of work? Well, I was lucky, I got hired through a temp service when I was a wet behind the ears 18 year old. Like any other kind of job, most people get hired by referrals. We hired between 3-5 people a year and never once placed an ad in the paper. It was all people who already worked there referring their friends.
I didn't say I agree with that practice but that's the why the world works, and also why it's so hard to find a new job.
I'd refer you but my company is closing, healthcare reforms and new supply practices where too much for us.
Good luck on your job search, good luck to us all.