"Happy birthday to you" is OK as a standby, but writing a personal birthday song for a friend adds a great personal touch. You don't need to be a great musician, and you don't have to be original. Simply taking the effort to write a little ditty just for your buddy will let her know that you really care .
Steal a tune. If you wish, you can borrow the melody from an old or obscure song that the birthday boy won't know. Alternatively, you can use the melody from a song that he will find funny or entertaining. For example, you may wish to pick his favorite song and rewrite it for a birthday surprise.
Think of an approach. A birthday song can be tender and sweet, or comic. You could make fun of your friend for growing older or discuss an early memory from your friendship.
Write the lyrics. Use the structure of the song as your cue. For example, if you were writing a happy birthday song to the Ritchie Valens tune "Oh Donna" for your friend Mellie, you could sing "Oh Mellie" every time you would normally sing "Oh Donna." You could also substitute the line "Happy birthday" for "Oh Donna" or use the same rhymes as the song uses.
Practice the song. You should sing it in a loud and comical way. It doesn't matter if you aren't a good singer; the lousiness of your singing will make the song even funnier.
Consider teaching it to your friends and singing it for the birthday boy together. Singing birthday songs as a group is always more fun.