Merrily

The B note is an open string (you don't hold down any strings). Remember the acronym to remember your guitar's string names:

(From the lowest-pitched string to the highest-pitched)

Eddie

Ate

Dynamite

Good-

Bye

Eddie

B (Bye) would be open 2nd string (B2):

The B note is placed on the 3rd line from the bottom of the staff:

Don't forget: the "hat" means two beats of silence. So, mute your strings when you reach the hat at the end of "Merrily"!