I don't like giving you a vague answer to this but math is truly vast so it depends a lot on what you're doing. Tell me more and I could give you more suggestions.

In my particular case, I started with Linear Algebra and that led me to some Applied Geometry and now I'm reading some (basic) things about Topology as well. I've also read a lot about Category Theory, in part thanks to HN as it keeps surfacing here often.