The easiest way to figure out the key of a song is by using its key signature. The number of sharps/flats in the key signature tell you the key of the song. A key signature with no sharps or flats is the key of C (or A minor).
How do I find chords based on notes?
Try using the notes of the G scale to find the notes of the G chord. Follow the same rules, starting with the root, G in this case, and then find the Third and the Fifth. Next, try to find the notes for F chord. Once you know a few chords, you can find a song that uses those chords and play along.
How do you know what key a chord is in?
For every note in the scale, there is a chord that starts on that note. The way to figure out what chords are in a key is to look at each note in the key and use a pattern to figure out each chord. The first note, C, gives us the C Major chord. The second note, D, gives us the D minor chord.