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"!