After you turn 18, you can begin to go on digs. There are many digs that have opportunities for young volunteers. There are always lots of excavation projects to sign up for, and so when you are ready, they will be waiting for you. Field experience is really necessary for a career as an archaeologist. The more field experience you have, the better are your chances of a successful career in archaeology.
Excavations are expensive, however.
But after the first year, you might get a staff position, and that would pay part or all of your expenses. This is where those clubs and leadership skills begin to pay off. The better you do on the dig as a participant and show leadership qualities, the better your chances of continuing but as a member of the staff. After university, graduate school will be your next stop. I won't spell out all of the details for you.