Go to the dead paper store and buy the textbook for the grade level you want to start with. This way you get a complete, comprehensive guide with table of contents that you can work through. Read, and do the sample questions.

When finished, buy the next grade ups textbook.

Learning math needs paper, it just does, don't try to do it online, it'll take twice as long and you'll learn half as much.

Be active not passive. Always learn with scribble paper ready.