1. Theres no wrong time to join a club (although its less awkward at recruiting time such as begining of term 1 and 2 because theres more new people)
2. Take a look at
http://www.msumcmaster.ca/clubs.htm and see what you're interested in; use search engines, look up the group on facebook, or as the other person suggested, email execs for information or to be put on mailing lists
3. I'd suggest you look in to faculty-specific club (like med-rad club, computer engineering, biomedical engineering, commerce, etc because they tend to be formed by classmates of yours and you can get a lot out of them, also look in to see if theres one based on your graduating year)
4. Some clubs have membership fees, some are free, and some teams require quite significant money investments to support the activities (training room access, travel expenses, uniforms, etc etc), so keep that in mind
5. The best clubs are free