If you signed up at directv.com for an account, you can pay online (one time or auto pay). If not, you can sign up usually with phone # on account. Worst case call 1-800-531-5000, push '0' repeatedly to get through to an agent quickly, and they can give you your acct #. All you need to do is call from the acct phone # or give them the # on account. Sounds more complicated than it is.
Depends on how you wish to pay your bill. If you already have signed up to DirecTV.com, no account number needed, just log on and you can make a one time payment no charge. You can use the IVR (However off the top of my head I think there is a $1.50 fee), just need your phone number that is assigned to your account. You can always talk to a rep (however to make a payment there is a $5.00 charge), you can ask them for your account number to make a payment and they should be able to provide that for you.
I know the directv system up and down. I can give you an answer to anything. I've been a technician for directv for over 4 years. Certified in every field.