If you have a topic you are interested in, well done! You've done what was the hardest part for me - you looked at topics outside of the school syllabus and thought "yeah I'm smart
enough to tackle that, even if it does look unfamiliar'.
Still, it can be hard to know where to go next. Stepping out of the classroom environment and into the wide world of textbooks, videos and websites can feel very overwhelming (and often an inefficient use of your time). Firstly, I would say that any experience teaching yourself material is INCREDIBLY valuable, whether or not you feel like you have learned as much as you would in a lesson. Those skills will be essential at university, and in life in general. But if you are looking for a bit of structure (as I still do), doing an online course can be a really great idea.
These courses, sometimes known as MOOCS, are essentially a bit like taking a module at university. Depending on the course, you might get homework, regular assignments, or lectures you should watch and take notes on. I have found some great ones in the last few years (I'll make a post on some more subject specific ones later), and I'm sure there will be an online course for almost any area of maths you want to cover.
Some advantages include:
Being held accountable to regularly checking in and working through assignments and grading
You are less likely to miss out important foundational concepts at the core of a topic.
You won't waste as much time trying to find the answer to very basic (and sometimes very obvious) questions on the internet
You will be connected with the other individuals doing the course, so you might be able to ask questions on the course website and get more immediate attention.
Many of these are completely free!
However it is also important to consider the disadvantages.
Although you may have to pay fees of anywhere from 10-30 pounds if you want to access tests and certificates. These aren't really necessary if you are diligent enough with testing yourself, so don't worry if you aren't willing to pay the extra amount, but keep this in mind
You might not get the flexibility that you want, and you may not find a course which is specifically on the topic you are interested in. The only thing I would say on this is that is often a good idea to get a broader understanding of a maths topic before delving into the specifics; no time is wasted by studying slightly different areas of maths, as often the same critical thinking skills can be applied
Some of the best sites for these courses include:
Stanford online https://online.stanford.edu/ (although this is only really good for compsci)
Go wild! Looking around can never hurt.