I will be starting a gym membership next week and was wondering, do people besides those creepy old dudes who walk around naked actually use the showers?

Gonna err on the side of caution and advise showering right after. All you have to do is carefully observe and you'll see the vast majority do NOT wipe down their machines after use if they sweat all over it. Place I go has towels and sanitizer pumps so I can wipe stuff down before/after I use but still.

Buy a cheap pair of flip flops at walmart for like 5 bucks to wear in the shower and one of those carabiner clips to clip onto the back/side/strap of your bag with bottoms facing outwards. Or just dry the top off and wear them out if its nice weather.