You can know all the notes on the guitar fretboard if you know these 2 things:
The string names
E A D G B E
The chromatic scale -
Most notes have sharps and B and E do not.
Start at A again after G#.
Just count up one fret at a time to teach yourself.
You can then memorise it while you're playing your favourite songs!
Watch the video to help this make sense! Thankyou.