If you're currently on a prepaid account you can change over to a postpaid account and get a postpaid plan, as long as you agree to and pass a credit check. Virgin Mobile will then set you up on a 30-day agreement. You'll also get to keep the phone number you had while on prepaid. Basically a 30 day agreement is like you're on a contract, but it only lasts 30 days instead of 1, 2 or 3 years.
If you'd like to switch from a prepaid account to a postpaid account, contact Virgin Mobile so a rep can do the change for you.