You want to go from your amp (which has your ipod/computer etc connected to it + L and R speakers) into the sub to give your hifi extra bass? You’d need to go from the “pre out” sockets on the amp (assuming it has them) to whatever input the sub has.

What’s the model number of the sub?

Btw if it’s a sub from a set of computer speakers then it’s not 500W RMS – this is the kind of rating you’d expect of a PA speaker (there are various ways for manufacturers to fiddle the numbers to make their products seem louder/more powerful on paper, it’s pretty common in computer and car audio). 20W RMS would be a more sensible rating for a computer sub.