You can declare your major on your Application For Admission.
If you are undecided about your major, Academic and Career Advising provides Tips For Choosing Your Major. In addition they offer Transfer Advising in person and through college fairs and workshops.
To change your major:
1. Sign-in to MyPage
2. Click on the Student tab
3. Under the Student Portal channel click on Request Major Change
4. Fill out the request and then click submit