If I’m trying to lose belly fat but gain muscle on my legs/bum what should I do?

Unfortunately you cannot spot reduce fat from a specific part of your body, it will come off with time.

What are your stats? (Height, weight, approx bodyfat% if you know it). In order to lose fat and gain muscle over time, you would want to be at a good weight for your height and maintain your weight, also called recomp. You want to follow a lifting program focused on your goals (Strong Curves by Bret Contreras is a good option), and get adequate protein, 0.8-1g per lb goalweight (you don't need to take supplements if you are getting enough from your diet).

If you still have weight to lose, you still want to lift and get enough protein bit you would eat in a small deficit. Muscle gain during weight loss is typically fairly minimal however.