Consult with your school's archaeology department chair. The department chair will be able to answer your questions regarding the requirements needed to major in archaeology. He can inform you of the necessary classes as well as potential career opportunities for a major in archaeology. If the department chair is unavailable, you should be able to consult with other professors in the department.
Declare your intent to major in archaeology with the College of Arts & Sciences or the office of the registrar. Usually, the school or college within the university that offers archaeology is the College of Arts and Sciences. If archaeology is another school or college, then proceed to their location on campus and register with them. Some universities require that declarations of intents to major in a subject go through the office of the registrar. If this is the case at your university, go there.
Study and attend class. Archaeology is not an easy major. It requires keen insight and forward-thinking. Archaeology is also a unique blend of science, social science and philosophy that requires its practitioners to use skills from a variety of disciplines. Successfully completing a major that requires such a broad range of skills does require hard work, discipline and desire.
Expect to perform fieldwork. Archaeology degrees will typically require some amount of fieldwork or lab work. Virtually all of the archaeological field depends on past, present and future findings, so fieldwork is a necessary component of majoring in archaeology. Also, more advanced degrees like a masters or a Ph. D. will typically hinge on doing new fieldwork.