Each note is assigned a letter as a name so people can discuss/write music. The letters corespond to the white keys of a piano. The notes of the black keys are named as sharp (#) or flat (b) relative to the nearest 'white key' note.

The letters go alphabetically with the notes in ascending order. That is, B is a full tone higher than the nearest A. C is higher than the nearest B. There is no letter higher than G, so the next full tone up from G is called A. The 'black key' note between A and B can be called either A# or Bb. The step from A to A# is called a semitone.

The range of notes from A to the next A, in either direction, is called an octave. This is because there are eight 'white key' notes from A to A. ('Eight' in latin = oct). Including the sharps and flats brings the total number of notes in an octave to 12. An octave is any range of notes that starts, and ends at the same letter. Like from C up to the next higher C, or down to the next lower C.

There's more, much more. Enough to fill many thick books. Keep at it. Have fun with it. Its a really good feeling when you hear a piece of music and recognise some of the theory that makes it work.