The M2121 pedals take a standard MTB cleat. Fortunately, you don’t even have to worry about any of that. The pedals are actually packaged with the cleats! All you need to do is buy these pedals and then some MTB compatible shoes. Sometimes, the shoes will come with the cleats as well, depending on the brand. This is handy because you will wear the cleats out. I have the M2121 and this is a picture of my setup.