The six open strings
From thickest to thinnest the strings are E, A, D, G, B, E. Say their names as you pluck them. Each fret raises the pitch by one semitone; on the low E string, fret 1 is F, fret 3 is G and fret 5 is A.
Use landmarks
First learn notes on the low E and A strings. Find the same note elsewhere with the interactive fretboard. At fret 12 the same note pattern begins again, one octave higher.
Do E♯ and F sound the same?
They sound at the same pitch on a normally tuned guitar, but the correct spelling depends on the musical key.