If I show up in London, ON can I just find a minimum wage, part time job.?
You may want to arrange in advance, depending on the job you want it may take you a while to find a job. Part time jobs are some what easier to find then internships. If you're looking for an internship in London, Ontario then maybe see if your school can help you find one. I live in Ontario but not in London.
I don't live in London so I cannot offer any personal experience but one potential problem you may encounter is that there are 2 large post-secondary schools in London ( Western University and Fanshawe College ) so you may have a lot of competition for summer jobs. It wouldn't hurt to try to arrange something before you arrive.
Where are you moving from?
What does ON mean ? Thousands here are looking for part time jobs, what qualifications have you got to offer. Have you got a work visa?