TL;DR My bachelor's degree didn't leave me with a solid education in mathematics and I'm not sure how to start a career in pure mathematics from there.
To start with, I'm 24 years old, living in Indonesia in Southeast Asia, and I just graduated from a low-ranking university (even for this country) with a bachelor's degree in computer science. I want to pursue a career in mathematics even though I've also enjoyed programming immensely (see all my posts on codegolf.SE) because I've loved mathematics for much longer and I feel like I wasted this first opportunity for education on a less-than-stellar degree. (Note: this less-than-stellar degree didn't even have a linear algebra course. Make of that what you will.)
I figure that a good path to a career in mathematics academia is to at least work towards getting a Master's and PhD in mathematics. The trouble is, is that even this is very broad. What topics/fields should I pursue (first) when looking for graduate programs? If I email professors, what do I tell them? "Hi, I'm looking for a graduate degree, but I don't even have an undergraduate's understanding of mathematics and I have no idea what path to pursue. Can you help me?"
Another hiccup is that I've just started a job as a software developer here in Indonesia to support my family, and I don't think I can leave that job to pursue academia for however many years, until I've worked there for a few years and saved up some money (especially if I pursue these degrees abroad). "Hi, I'd like to join your graduate program, but not till 2023." doesn't seem like a great thing to receive in your email.
This delay does give me time to give myself an informal refresher in undergraduate mathematics, but I have no idea where to even start. Read a textbook? Join an online course? Find a tutor here in Indonesia?
This is not to mention that I'm not sure how on earth I'll pay for everything, or that for most master's or PhD courses (or a combined degree) expect you, quite reasonably, to have a bachelor's degree in mathematics.
And lastly, my main mathematical interests are in number theory, algebra, and calculus, in approximately that order. I was thinking of applying to a North American university (maybe the University of Waterloo in Canada) before applying to Australian or European universities. I have US citizenship, which should help a lot.
In short, I have a myriad of choices and a bad starting position. I understand this is a very general question, so I thank you for any advice you can give or resources you pass along.