Watch the Note Names: 6th & 5th Strings online guitar lesson by Jeff Scheetz from Street Theory for Guitarists
There are a ton of notes all over the guitar neck, and while it's cool to know them all, I first want you to memorize the notes on just two strings: the 6th and the 5th strings. These are extremely important. Many of the chords, scales, and theoretical concepts in this course will be using root notes on these two strings.
The root note is what names a chord or scale. The root note is important. While you can have root notes on other strings, most of the things you'll play will have a root on the 6th or 5th strings, so it's really crucial to memorize these.