Happy Birthday 

You can view the song in the key of D, G, C, A, & E below.
Try playing the song using the Roman Numerals 
and thinking of the chords as numbers: 1, 4, 5 (or I, IV, V).You

Start by playing:
1) Whole Notes - one strum per measure
2) Half Notes - two strums per measure
3) Strum Pattern