What you might wanna try in the mean time: sing the first note in a particular melodic phrase as it sounds in your head. In the beginning, you may have to sing a phrase until you get to a sustained note. Anyways, while you're singing this note, locate it on your instrument. This part may take practice, I used to start singing the first note I tried to play, and would have to start over. After you figure out what this note is, try a few chords that contain that note. Try a few chords that are from a key with that note. See how it sounds. Figure out more notes, from different parts of the song. Analyze those notes to figure which keys you could possibly be in. Sooner or later you'll start intuitively knowing which chords to use.
Make sure not to let keys and theory box you in, and have fun.