In many states, you will have to affirm that you cannot vote in person for physical reasons such as age, disability or being physically absent from your state that day. States may require a doctor's signature or just your signature verifying your reason is truthful. To find your local elections office's information, go to the Election Official Directory. Choose your state and region to find the contact and address of your local election official or the link to your Local Elections Website.
If you are out of the country or in the military there are specific voting rules that apply to you. If so, designate that you are an overseas or military voter as an option during the Voter Registration or Absentee Ballot Request process.