Two strings worth memorising

Walk up the thickest string one natural note at a time: E, F, G, A, B, C (mi, fá, sol, lá, si, dó).

Notice that E to F is one fret, and G to A is two. The neck is not evenly spaced in note names, because the alphabet is not evenly spaced in semitones — that is the same B–C and E–F you already know from the keyboard.