If you only have one speaker, you will only be able to output mono sound. Stereo sound requires two speakers. By convention, you should use the left 1/4 in. output to run to your speaker and ensure that the PAN knobs are all moved to the left (since you aren't using the right output). It is also worth noting that unless your speaker is powered, you will need an amplifier to run sound through the speaker.
Stereo only works with two speakers, but it doesn't really matter as you can do a mono mix just fine. Stereo is useful for spreading out the apparent source of inputs, but it isn't by any means critical to making something sound good, just a helpful and optional tool. Whenever you do decide to get another speaker, you'll be able to plug it in through the right output and then the pan knobs will determine how much of the output from each channel goes to either the left or right speaker.