Starting right now, commit to Never being late on a bill payment (of any kind). If you do this for at least 6 months your credit score will rise. Also, take every extra dollar you have and pay off all your debt. Refuse to buy another thing unless it is absolutely necessary.
I would strongly encourage you to sign up for a credit monitoring service, offered by one of the three credit agencies, that will let you check your score each month. It will also offer you tips on raising your score. Try to keep your credit limits as low as possible, and pay off any delinquent debts ASAP.