how to know waht kind of computer can be installed in my PC?

hi everybody here:
I wanna order a new HDD for my PC. I'm get confused because there are
a plenty of kinds of HDD with different brand, different capacity, diferent discription. I don't know what's ATA, what's SATA......
could anybody give me any instruction on how to get a HDD fit for my PC?
it' too difficult for me to solve this problem
thanks in advance

do you know what hard drive is in the machine at the moment?

what make and model PC is it?

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