Which one is this? — the register problem

The musical alphabet only has seven letters, so every letter turns up more than once on the staff — a lower one and a higher one, an octave apart. "B" isn't an instruction until you know which B.

Two B's

50 bpm

Same letter, an octave apart. The low B is on the G string, below the staff; the high B is on the middle line, 1st finger on the A string.

Two C's

50 bpm

Middle C, 3rd finger on the G string — and the C an octave up, low 2 on the A string.

Read the position, not the letter

52 bpm

Two B's with an open D between them. The staff shows you which is which — one below it, one on the middle line.

The usual mistake

Seeing "B" and playing the first B your hand knows, ignoring the octave the staff is showing. The letter tells you the note's name; the height tells you which one.

About "C4" and "C5"

Later you'll see notes written as "C4" or "C5" — just a precise way of saying which C (C4 is middle C). For now, "the lower one" and "the higher one", read from the staff, is enough.

Try

Play low B (G string), then high B (A string), then low B again. Same name, clearly different notes. Then do the same with the two C's.