To help with some of the math required by general relativity, there's a good series on YouTube by the user "eigenchris" on tensor calculus, which I found helpfu

To help with some of the math required by general relativity, there's a good series on YouTube by the user "eigenchris" on tensor calculus, which I found helpful for a geometric intuition of what's going on. He also has a series on GR itself, which I haven't watched yet. If you're interested in learning GR and want specific information on the area of differential geometry you'll need to understand it, then this series is a great start.