If you don't have a mono switch, you will, at a minimum, need a Y connector that combines the left and right signal into a single, summed, signal to feed the single amplifier (or channel) that will be powering your one speaker.
I have heard a number of mono systems, and using a transformer to sum the two channels sounds better than a simple Y connector. Of course it is a more costly approach that I would recommend for someone doing a permanent mono system and not something that is just temporary fix. A local dealer in my area has a tech who has built a comparison device that allows one to switch between Y connector and various choices of transformers so one can hear what each transformer does to the sound. I am not that big into mono, but, I was surprise at how good a mono system can sound, particularly when care is taken to optimize the sound, including using the right transformer for the job.