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 first 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 uses the notes B, C and D on the second string. Practice naming the notes out loud as you play, and also counting as you play. The numbers under the TAB staff are bar numbers to help you remember where you are in the music.