If you're born out of Thailand youll need to ask your mum if she registered you at the thai embassy in the USA. If she did, you request you birth certificate for them and if you have never had a thai passport. They can issue you one. Then you have to come to thai to get a thai id. Once you get here you need to get a thai id to finalise your citizenship. Thats what I had to do about 7years ago, from Australia.
Email your local thai embassy, they should be able to help you