If you are in the process of building credit and you are really serious about proving your creditworthiness to the moneylenders out there then single most important thing that you need to know is...
Always make the minimum payment before the due date.
Do not let a month go by without making at least the minimum payment on your debt because for every month that you are late is a new stain added to your credit history.
Did I mention interest and late fees?
If you are sending the payment by mail please do not wait until the due date to send it out.
Send you a minimum payment and a couple of weeks or at least a week before it is due, this way you know for fact that the lender that his money on time every time.
The biggest secret about credit: Pay your bills on time every time.