We don’t know what you like and dislike. You should consult a travel agent or do some research.
To develop an itinerary yourself, watch a bunch of YouTube and/or TikTok videos and read free travel guides you find on Google. Start searching with something general, like “Thailand vacation.” Make notes. When you see somewhere you like, YouTube and TikTok will suggest more videos. Once you have a list of places, plot them on Google Maps and use the Rome2Rio app to determine your best way to get from one place to another. Then, post your rough itinerary here for comments. Once you have a final itinerary, use the skyscanner app or Google Flights to select airline flights, then book directly with the airlines in case changes need to be made. Use the Agoda or Booking app to select hotels (each one has hundreds of reviews), then book directly with the hotels in case changes need to be made.
Or, ask Facebook and go where a few random people like to go and stay in hotels one or two random people like.
Don’t buy any of the travel guides on Amazon recommended by spammers. They're worthless to the point they have no reviews or a few fake reviews before they disabled reviewing. They are just a legal way to scam you out of money.
In Thailand, the ATMs dispense the cash, then the receipt, then return your card. Many people have walked away with the cash and not waited for their card to be returned. When that happens, the ATM sucks it back in and you have to call the number on the ATM to get your card back.