it doesnt literally mean "am" lol but i dunno hw to explain it but ya use it at the end of a sentence XD
desu + ka = question
ka = ?
and its watashi wa nihonjin desu
watashi wa = i am
nihonjin = japanese
desu = stating what something is like "to be"..."to be half japanese"
im a bad teacher. I've taught a 20 year old japanese dude english though lol