Hi, I want to buy a new Theremin the next days, and wanted to know what things I need so that I can play it. Has it got built-in speakers, or do I need to buy 'em? Any cables or sth else I need?
Thx for helping me 
You’ll need a straight mic stand (to mount it), a 1/4" instrument cable…maybe 10 ft long, and you will need an amp. No it does not have built in speakers. You could run it into a mixer and thru a PA. You can find all those things at a big box guitar store or on your local Craigslist.
You may want to do a little research before buying an instrument,
Money.
Sorry I had to 
Mhuxtable has said it all. if you can try one, go. It’s not a common instrument.