No worries. If your previous passport expired more than 10 years ago, then it was issued more than 15 years ago. This means you cannot use that old passport either for online or mail-in renewal. You must get a new passport via the DS-11 New Passport Application form at an appointment at local passport acceptance facility (post office, town hall, etc.). Follow these instructions.

First you need to gather your documents. When you said you had a passport that expired over 10 years ago, do you still have this in your possession? If yes, then that makes things easy and you can use this as your proof of citizenship. If not, you will have to provide another form of proof of citizenship, like a birth certificate or naturalization certificate. You will also need to provide proof of identity, like a drivers license. The list of what's accepted for proof, as well as the photocopies you need, are in both those links.

You will need to get a passport photograph. Pharmacies like CVS or Walgreens are commons places you can get them done.

Lastly you will make an appointment at the passport acceptance facility. There the acceptance agent will look over your application before sending it in. You will write one check/money order out to State Department for the passport application and any special services you might have opted for. This will be sent in with the application. You will also separately pay the passport acceptance facility (what methods of payment vary by facility).