Visa to Thailand
You need to obtain a visa before you travel to Thailand. Acquiring visa is not difficult for Czech citizens, though. All you need is a passport valid for next 6 months after you return and at least 2 empty pages.
You can apply for a tourist visa for example via Thai embassy (for example in Prague). The above mentioned tourist visa is granted for 2 months and can be extended before it expires on the Immigration Bureau in Bangkok.
You might even use the possibility to apply for a visa on one of the international airports after your arrival. This is called "Visa on arrival". You need to fill in a form, enclose one passport photograph and pay a fee of 1,000 bahts. The issuance takes 10-20 minutes.
Warning! "Visa on arrival" serves just for 1 entrance! You can enhance visa for 1 entrance into visa with multiple entrances on relevant office in Bangkok when you pay the amount required. However, you need to deal with this in time to avoid an unpleasant surprise when you return from a trip for example to Cambodia and find out that you are restricted from entering Thailand again.