The first is to apply for a non-immigrant O-A (retirement visa) visa prior to arrival in Thailand. The second, and perhaps easier option, is to enter Thailand on a different form of visa* and then change your status. Subject to meeting certain criteria, there is a procedure by which you can then extend your stay in 12 monthly increments on the basis of your retirement.
The basic requirements for the granting of extension of stay on the basis of retirement are that you:
The asset and income requirements are simply to ensure that you can adequately support yourself, and will not become a drain on the Thai community.
You must be able to verify that you hold: