Where to start - degree

I'm fascinated by astrophysics and I'm considering pursuing a degree (passion project, not as a career). I'm 40, and thus 22 years out from high school maths. Where do I start to build up the knowledge in order to eventually pursue a degree (bachelors, then masters, and I don't die of old age first, a PhD) in astrophysics?

I was very, very good at maths in school, but it's a long time ago, and I'm a bit rusty.

Thank you in advance!