it's simply the length of time you spend in Thailand without leaving the country. If you stay here for 90 days, you have to report. It doesn't matter whether you have an apartment or not. If you leave and come back, the 90 day limit starts from 0 again. Example: you arrive on 1 January. You leave again on 25 January and return on 1 February. You stay 90 days: report on 1 May (90 days from 1 February, i.e from you most recent passport arrival stamp). Always 90 days after your last arrival. You can report up to 15 days before and 7 days after the end of the 90-day period - in the example I gave above: 16 April to 8 May). Is that clear?
You can do it online. You only have to do the first one after entering the country (i.e. 90 days after entering the country) in person at the immigration office in the province you live. The online registration is easy.
PS: Thailand "of all places"? Would you suggest a better place? I'm here because I love Thailand and Thais. I'm not some kind of globetrotter with millions of dollars 🙄 I'm actually very happy in Thailand.