Ledger lines
The staff only has room for about an octave and a half. For notes above or below it you draw ledger lines — little extra lines, one per note as needed. Nothing new to learn: the same step-and-skip rules just carry on past the edge.
Below the staff — most of the G string
Middle C sits on the first ledger line below the staff. Keep going down and you're on the G string: B just under middle C, A on the next little line, open G hanging below that. In first position the whole G string reads below the staff — normal for the violin, not an exception.
Above the staff — the top of the E string
Anchor from the edge
The trick with ledger lines: don't count from the middle of the staff. Count from the nearest open string — open D for everything below, open E for everything above.
The usual mistake
Freezing at ledger lines. They look like a different language and they aren't — B on the second little line below is just "a skip under middle C", the same as any other skip.
Try
On the G string, play open G, A, B, middle C, and back. Watch the notation as you go — four notes, all below the staff, all steps. Get your eye used to travelling down there.