My first time building a PC and was wondering if I need to buy anything else I've got the OS and keyboard and screen and mouse
The rest: https://t.co/Ykczv0iet8
It should come with the cables. The motherboard should come with a couple of sata cables to connect the hard drive, the psu contains the power cables needed for the drive(s) and motherboard, the case includes the cables needed for the front ports and power button. You mentioned you have the os, if it's a disc you'll need an optical drive (cd/dvd rom).
You might add speakers if there's anything you wish to listen to coming from the pc unless you already have them or they're built into the monitor or something.
It should come with the cables. The motherboard should come with a couple of sata cables to connect the hard drive, the psu contains the power cables needed for the drive(s) and motherboard, the case includes the cables needed for the front ports and power button. You mentioned you have the os, if it's a disc you'll need an optical drive (cd/dvd rom).