There are some jobs on campus, but not tons. Your best bet is applying for the McWork/Work-Study program through Career Services (I think it's called the Student Success Centre now). I haven't done this for a few years, so my information might be a little dated.
The jobs are mostly office stuff. It can be administrative like answering phones, it could be a research assistant position, it could be a position in the hospital like cleaning lab animal cages, etc. Lots of the jobs are for Engineering and Health Science students, but there are definitely some open to everyone.
You need to be eligible for OSAP to get these positions. You don't have to have
received OSAP, just be eligible for it. If you haven't applied for OSAP, you can fill out a budget information sheet from the Financial Aid office and they will determine if you are eligible or not. It asks how much you make during the year, how much your parents make (unless you're more than 4 years out of highschool), what you spend your money on, etc.
Here's some more information:
http://sfas.mcmaster.ca/work_study/jobs.html