It's easiest to meet people during the first few weeks when everyone is new and also feeling a bit lost. I met lots of new people during fresher's week when you are forcibly and cringingly introduced to other students. In fact two of my best friends now were introduced to me on a freshers' tour of the university!
I joined a few societies and did some campaigns with Amnesty and also some voluntary tutoring. Voluntary tutoring was great because I had been thinking about teaching when I graduate and it gave me a chance to do that, while helping someone else. I also got involved in the Students' Union. For a lot of students college is more about societies and the union than the actual course.