I have pimples mostly on my butt and inner thighs. What do I do?

This might not be acne at all.

Stop wearing synthetic materials. If that doesnt fix or at least improve it then you might be dealing with Hidradenitis Suppurativa which is autoimmune. And that means you likely have something in your diet that is setting it off. Sugar and dairy are the most common but really any of the big 5 are possible. Which are dairy, soy, egg, grains, and night shades. I would look at digestive function too.