@Jean *****
then you can go to the main immigration office by the airport or the one over at Central Festival Mall.
After that 30 days runs out you can go to the border, stamp out of thailand, stamp in and out of another country and stamp right back into thailand to get a NEW 45 day visa exempt entry (which you can also extend for 30 days as well)
The closest land border that's open for foreigners to cross is the Lao border at Chiang Khong in Chiang Rai province. It's about 300km away from C/M.