Start with whatever camera you currently own.
Becoming a better photographer is ALL about having the right knowledge and then knowing how and when to apply it. It is definitely NOT about having better kit!
If you don’t own a standalone camera, then use your Smartphone. The principles you will learn will improve your shots regardless.
If you’ve got a Bridge, Mirrorless or DSLR camera then this free online photography course will help you to understand it better.