Where are the "instrumentals" you're listening to? Are they other tracks in the same Cubase project?
I dont know Cubase but it sound like one of a few things, are you recording with headphones on? If not you'll be getting bleed into the mic, actually other than that im not sure, what "input" do you have selected on your vocals channel?
i suspect that you have a soundcard designed for gaming not an audio interface... and there's a tab marked something to the effect "record what you hear"...
Where would I find such a tab? In my control panel or within Cubase?