I'm going to assume you want the requirements for the re-entry permit:
A completed TM-8 form
Either 1,000 THB for a single entry, or 3,800 for multiple entry
Passport photo (4cm x 6cm)
Photocopy of passport (main page and latest entry stamp)
Having a re-entry permit *DOES NOT* give you the right to leave and return; you will need a Certificate of Re-entry, and you may be outside of Thailand for a few months.
You can also get a re-entry permit at most international airports as well. At BKK, the re-entry permit desk is open
****
.