What are the rules that a self-learner should follow to ask good questions in this site?
In my opinion, almost all questions that a self-learner is likely to have would be better on Math Stack Exchange. These include all questions about mathematics itself, as well as many questions on how to learn mathematics best.
That being said, there are some questions which are more about pedagogy than about mathematics itself. As a self learner, part of your job is to design a curriculum for yourself, and curriculum questions are certainly on topic here. It would also be on-topic to ask questions about pedagogical choices made by textbook, i.e. why are things presented in a certain order, or why are certain topics included.
So, here are some questions I can imagine a self-learner asking that would be relevant here:
What kinds of mathematics would be helpful for an architecture major?
Does it make more sense to learn linear algebra before multivariable calculus?
Why do books on trigonometry start with right triangles, and only move on to the unit circle later?
Why do calculus textbooks cover numerical techniques of integration?
What parts of math should I review before taking a calculus course?
This site is for people who teach mathematics to ask each other questions about the best way to teach. If you are learning mathematics by yourself and you want to ask questions about mathematics, then you should ask at https://math.stackexchange.com/ rather than here. If you are in some other situation, then you should explain in more detail.
As Niel has pointed out this site if for questions about mathematics education. We are interested in, for example, ways in which to improve mathematics education. We are not about mathematics per se, but the subject is teaching/communicating/learning mathematics.
If you are trying to teach yourself various theories of learning and you would like to ask about how this might be applicable for mathematics, then this is just the place to be. If you are interested in methods of self-learning, then I would also say that you have come to the right place. I don't know that there are any specific rules that you need to follow. As long as you stay on-topic then that is fine. In general you cam ask questions about anything that you have thought about and would like an answer to. So, if you have a question about ways to teach yourself mathematics, I think that would be a great question. You might, of course, want to be a bit more specific. So, maybe you could ask a question about good books for teaching yourself group theory. Or you could ask for how time is best spent in trying to teach yourself a topic.
If you are trying to teach yourself mathematics, then math.SE would be better since that site is specifically about mathematics.