Thailand / Myanmar (Burma) the legal international land border crossings are:
1. Mae Sai (T) / Tachilek (M)
2. Mae Sot (T) / Myawaddy (M)
For bikers however, these are not viable options for touring Myanmar, as travel permits for individual riders are not yet generally available, and who knows when it will become a reality?
You can get in, going via a tour company, buying a full package – guide, back up, hotel & pre approved route; but you can’t just rock up to the border & ride in.
You’re best bet maybe to fly in & rent a small bike locally.
Check out this 1st GTR ride into Myanmar in 2013
Note (1) that there is a 3rd legal crossing from Ranong (T) to Victoria Point (M), in the south of Thailand. This however, is not a land border crossing, but a sea crossing by boat.
Note (2) that in August 2004 Simon & Suzi Harby riding an Africa Twin & a Transalp, rode from India via Nagaland & Manipur, thru Burma & into North Thailand at Mae Sai. This was a world first, but is most likely going to remain a one-off trip & not be repeated for a long time. Check out Simon & Suzi’s site for info & their trip report. Also take a look at this GT Rider forum report.
Take a look at the Borders Directory and Borders Map for guides to legal international land border crossings in the area.