I can anti-recommend Brilliant. It might be okay if want to pay for a subscription to solve puzzles but it's not a good environment for learning. In your case

I can anti-recommend Brilliant. It might be okay if want to pay for a subscription to solve puzzles but it's not a good environment for learning. In your case I would start with Khan Academy - they have material from kindergarten level all the way up to undergraduate university level so you can pick things up from where you feel most comfortable. And Sal explains things in a very human way which is nice for people who have struggled with math before.