Tourist visas in the USA for Thais are very difficult. Fiance visa does work...if you are serious about marriage. Don't do it otherwise. Go visit her. It took my wife and I several years to complete the Fiance visa, marriage, and citizenship. Good luck.
You need a valid passport (always on international flights, ALWAYS). You should have information about where you're staying. I recommend travel insurance, but not required. No COVID test or vaccination required.