I've read most of the questions on this website about this matter, but I'm still confused. I pulled off the handle and wall plate. But what do I do after that?

I see a small (4 inch approx.) hole in the wall with a "unit" that looks like a cartridge that holds the handle. I can turn it to hot and cold, but that's about all I can move. What do I need to look for to clean out any possible garbage that may be stopping the hot water from reaching the shower? And yes, the faucet at the sink does produce very hot water!

If you can describe to me what I need to do - step by step, I would greatly appreciate your help!! I am REALLY tired of taking luke warm showers!!
THANKS!