Start a conversation with a classmate. Students always chat with each other before class, so show up a few minutes early and strike up a conversation with someone. It can be on the reading material, on the class in general, or it can have nothing to do with the class. You can even contribute to someone else's conversation if you hear an opening.
Join a sports team. Sports, especially team sports, are a great way to meet people. There's camaraderie and usually downtime you can use to chat with your teammates. Many intramural sports leagues will have something called a "singles team," which is a team made up of people who don't have a team. Otherwise, inquire with the league officials about teams that need players.
Join a club or activity group of some sort. If you enjoy reading, consider joining a book club. If you like hiking, think about joining an outdoors club. Whatever your extracurricular interests are, try to find a group that explores those interests. This common bond can be a great way to make friends. You can look for these groups through the school, but websites like Meet Up are good too.
Involve yourself in a cause that you feel passionate about. Maybe it's global warming or a certain political party. Perhaps you feel strongly about literacy or drug decriminalization. Whatever your opinions on the world are, it's a safe bet that someone else out there shares them. Try searching on the Internet for your cause and your school or the name of the city in which you live.