Depends on the office you use but generally it will be a copy of your passport. A copy of your latest entry stamp and a copy of your visa (if you had one), a photo and a printed TM30 filing that shows you are staying in the province you’re applying at.
Again depending on the office you may be charged something like 200-500 baht.
There are 3 proofs that could be asked for. Cash of 20,000 baht or equivalent. Cash only. A bank statement or phone app won’t suffice. Proof of an exit ticket and proof of accommodation.
Rarely would any of that be asked for unless they are looking for a way to deny entry.