To play the note B, pick the open 2nd string.
Play the C note with the first finger of your left hand just behind the third fret of the 2nd string.
Play the D note with the third finger of your left hand just behind the third fret of the 2nd string.
Notes written above the middle line of a staff usually have their stems going down. Notes written below the middle line of the staff usually have their stems going up. The stem for the B note can go up or down.
Here is an example which demonstrates the notes B, C and D on the second string.