How can I prevent unauthorized in-app purchases on my iPhone?

So I was on vacation and my brother used my phone and asked if he can download load some games.. he would up making hundreds of dollars of in app purchases some for 50$ 60$ 40$ 20$ 100$ multiple player games were they shoot and said he was okay but now what’s the best way to clean please help course they really

[Re-Titled by Moderator]