Here is my advice regarding music theory:
Understand that music in the European tradition consists of 12 notes per octave. The thirteenth note's frequency is double the frequency of the 1st, that's why they are in perfect harmony and have the same name (e.g. a and a'). A major scale consists of the notes 1, 3, 5, 6, 8, 10, 12, 1', 3', 5' etc. You can start on any note and follow this pattern to get a major scale. Other types of scales have other patterns.
Now, everything becomes super-complicated because over 9000 years ago some retard invented a notation system pretending that all the steps in a major scale starting with c are equally wide. Since then we are forced to use #s and bs to refer to the missing notes of the 12 steps when playing major scales that are not c major.
When music theory and notation seems complicated, always remember that the patterns are much simpler when thinking with 12 notes and be aware that the traditional notation system is very stupid. Thank me later!