A college student registers to vote in Virginia the same as any other applicant: every prospective voter must submit a voter registration application (PDF). Remember, you must update your voter registration information whenever you change your residence.
Every voter in Virginia must submit their residential address when registering to vote. (A mailbox cannot serve as a residential address.) If you are unable to receive mail at your address, you must also submit a local mailing address. A dorm or college address can be an acceptable residential address and does not disqualify you from voting.