The sound of castanets is instantly identifiable - outside Spain at least! - as flamenco.  Flamenco purists dismiss castanets as not being "authentic", but in fact, there are a lot of things about modern flamenco that aren't authentic - so why pick on castanets?

Look carefully at the hand position in this clip:

If you want to play castanets well, you must get the hand position correct, right from the start.   Most beginners start out all right, then quickly let their hands fall into a more comfortable - but less efficient - position. Get into that bad habit and you'll never master castanets!

The other mistake beginners make is to have the strings too loose.  Sorry, but you should have a nasty red groove in your thumb when you take off your castanets!

Finally, don't be tempted to buy cheap castanets just because you're a beginner.  Many cheap castanets are made for tourists, not for real dancers.  They're often too small, and even the correctly sized ones sound dreadful.  If you buy a good pair of castanets and decide to give up dancing later, there's a good chance you can sell them on to another dancer.