To self-learn, pick up a book, or 2 or 3, on the subject, and read a few pages at a time, maybe at first in a cursory fashion, to have a rough idea of what it is all about. (Let me refer to maths, to be specific.) If this is an entirely new topic, maybe many of the ideas are new to you, and you may not have a clue as to what it is all about. (The smarter students may have a reasonable grasp of the main idea). Don't worry - that is only to be expected. Read again, this time more deliberately. Try your best to understand the main ideas. If there are a few worked examples, study them carefully; often this greatly furthers your understanding. Maybe read another time. Then try some of the simpler exercises; if you can manage, try the harder ones. If you cannot do these, read again and see if you now understand a little more. If not, read another book on this topic. If after a while, you still do not understand a few areas/concepts, ask your friends, teacher etc. Keep trying in this way, keep going thru the various topics, until you eventually master them. If after lots of serious efforts, you still don't get it, then maybe, if you can afford it, get a tutor.