The y cable will connect both channels together so that the signal going to the sub, left and right channels will be the same, effectively mono.
Either get an amp with dedicated sub outputs, which should be electrically isolated from the main pre outs (if the receiver has them) and use the y cable, else you will need a mixer to blend the left and right signals to give you a mono feed to the sub.
Either get an amp with dedicated sub outputs, which should be electrically isolated from the main pre outs (if the receiver has them) and use the y cable, else you will need a mixer to blend the left and right signals to give you a mono feed to the sub.