In order to hook up your subwoofer properly, a “Y” splitter cable is not needed, just a single RCA cable.
You need to connect the single RCA cable from the subwoofer output on your receiver to the Left/LFE (white) RCA input on the back of the subwoofer.
I recommend using a subwoofer cable. It will be better shielded from noise and make for a better signal.
Any cable designed for subwoofer use will be fine. It doesn't have to be expensive, the cheaper ones work just fine.