dude, juz learn happy birthday 1st.. i feel you are doing a big jump as you dun even understand the meaning of key of G..
do happy birthday in the below ways:
-try many different keys to play that song
-try to sub jazz chords like 7ths 13ths n such in..
-do in many different strumming pattern
-learn to pick the melody out in all different kinda picking method
-change the whole progression to minorish
-understand how a major n minor chord, how they work
there are just way damn many ways you can learn from a simple song like this.but unless you just wanna have fun learning songs from tabs.