When you apply for the US tourist visa, it is important to prepare the documents such as passport, ID card, proof of employment, proof of having adequate funds for the trip such as bank statement or updated bank book, copy of previous visas to other countries, proof of accommodation during your US. It is not required to book a flight when applying for the visa. Also, it is important to know that you have strong ties in Thailand such as employment, property and assets, and family who is depending on you.