Thanks for your advice Simon, I appreciate it. Bee has tried only speaking Thai with me. It works for a while and then he gets frustrated and reverts to German (he speaks English, too, but German is easier for him). Yes, I need a school so that I can learn the rules and the alphabet/writing system.
From my experience, Thai people's level of English is extremely low. It's not their fault of course, as the education system is very badly funded and only people with money can get a decent education.
In any case, I am the foreigner here so I should learn their language, not the other way around.