chords are the key. as you will soon learn, it's all been systematised long ago. the system of chords determines how a song sounds. it will also determine what notes are used for the melody.
so, you have to learn to play some chords as the starting point.
although there are many places to begin, i suggest that you just do this: pick up a simple guitar song book that contains its own chord charts. choose a song you know. play the chords as best you can until your fingers bleed. sing the song (quietly in the bog when no one's home).

when you've done that a couple of times, you're a guitar picker! and pretty soon, you will know a song. that's a great beginning.

the rest is just getting about better and better. and it takes time, but you've got time, right? you're going to be playing for the rest of your life, right?