It's important to note that stereo plugs (2-channel) are distinguishable from mono plugs (1-channel) by the number of black stripes on the end of the plug itself. If it has a thick and thin line (thicker towards the end), it's stereo. If it has only one line, it's mono.

If you want to bring a source from stereo to mono, don't try to use a mono plug, because you will often only get one of the two channels (for instance, only left), instead of a mix of both. Instead, use a specialized adapter, something you can probably find at Radio Shack or some similar electronics or audio hardware store.