How much can you make? It depends on how much you want to put into it. If you want to do it part-time, you can get about $300 - $400/month (15 jobs/week). If you want to do it full-time, you can expect $800 - $900/week (50+ jobs/week). However, not all MSCs pay on the same schedule. Some MSCs may pay a month behind, some pay the last Friday of each month, and some take 60+ days to pay. MS is not for "quick cash".
What are the jobs? It varies from region to region, MSC to MSC, and your experience. If you are just starting out, you are going to be getting the cell phone, fast food, gas station, and retail visits. Before you get the higher paid shops, you need to establish a great rapport with your MSC. Be the rock they can count on. If a scheduler calls/emails/texts you and asks to get a job done, tell them sure!! I did this with one MSC for a few months. I was grinding out the tedious jobs that nobody wanted to do, and I would accept any job that came my way. Eventually that MSC called me and asked if I wanted to try out a hotel shop. It ballooned from there.
Where do you find them? On your MSC job board. HOWEVER, you are competing with other MS'ers in your area. There are lurkers that will camp on the job boards, refreshing until they see their favorite shop, and they will pounce on it like a hyena. Your best bet is to follow the advice others have given you. Camp, refresh often, and be the first to respond to emails and calls.
A bonus tip: If you are one of those people that do not answer phone numbers you do not know, stop it!!! Every call can be a potential MS recruiter with a job. Remember, you are building a solid portfolio with your MSC, so answer those calls!!!