There are many ways to be involved while in college. I personally joined a fraternity my second year, was part of a volunteering organization, and worked in several research laboratories. Many universities have people flyering and bulletin boards with activities. Look out for them and see what you like and go for it!
In general, I think you will find that it isn't very hard to get involved in campus activities. The volume of student groups will vary from school to school but the energy with which these student groups reach out to students, particularly first and second year students, is pretty high. Faculty, staff and student bodies look forward to bringing in new minds and ways of thinking and one of the ways of encouraging new students to stay and contribute to the school culture is through student groups.
As far as "the best way" here are three ways that I can think of 1)pay attention to flyers and on-campus advertisements during the first 14-30 days of school. 2)Look at what is posted in popular areas of campus like cafeterias and libraries. 3)When you feel comfortable, talk to other students and/or faculty about interesting groups that exist on campus or groups that are small and need a boost of energy :).
Best of luck to you!