Commit to accepting social invitations. In order to make friends, you need to interact with people in social situations. If you're introverted, it's hard to go out on your own. You may also have a tendency to turn down invitations if it seems like you may not like the activity. Make a commitment to accept every social invitation that comes your way, as long as you don't have other plans. This forces you to be more open. Even if it's an activity that you didn't enjoy, you still had the opportunity to meet new people.
Join extracurricular activities. The key to making good friends is to find people who share your interests. When you join clubs that focus on something you enjoy, you're likely to meet people who are like you.
Talk to the people in your residence hall, if you live you on campus. If you live in a dorm room, keep your door open to signal that you're open to talking. Smile and say hi to the people who walk by and be friendly when you are walking down the hall.
Make friends online. For those who are shy or introverted, the Internet can allow you to be yourself without a lot of social pressure. Join forums for students at your school or in your town. Once you find someone that you click with, invite her to do something in person.
Meet the friends of your friends. When you have a few people that you like and can connect with, you can become a part of that person's social circle. Ask him to invite you to parties and other social outings, so you can meet more people.