What are my options for studying mathematics online?

In the next few years, I’m looking to start a PhD in applied linguistics or language teaching.

I’d like to do some pre-emptive work on my maths – I will obviously need statistics. However, I also have a layman’s interest in complexity theory, and would ultimately like to explore that in relation to language learning and language change.

I haven’t formally studied any maths since I was 16. I wasn’t unsuccessful at it, I was just uninterested at the time.

I don’t need a bachelor’s degree, although I will consider it.

So… What are my options? Concrete recommendations and specific courses will be welcomed!