MOST songs contain three main chords (and maybe their relative minors).
If you are playing in G, the major chords will be G, C, D(7).
The relative minors for the major chords are G (Em); C (Am) and D (Bm). In some places you may be able to use sevenths so's they don't make it sound discordant.
So, let's say we want to do "Go, Lassie, Go" in G. (The chords are from a site I went to.) In line one for example, instead of the first Am play a C. In the last line instead of the first G play an Em
Oh[G] the summer[Am] time is com[G]ing,
and the[C] tree's are sweetly[G] blooming,
And the[C] wild mountain[Em] thyme,
grows[C] around the[Am] blooming[C] heather,
Will you[G] go[C] lassie[G] go,
and we'll[C] all go to[G]gether,
To pluck[C] wild mountain[Em] thyme,
all a[C]round the [Am]blooming[C] heather,
Will you[G] go[C] lassie[G] go.
I will build my love a tower,by yon crystal fountain,
And on it I will pile,all the wild flowers of the mountain,
Will you go ,,,,,,
If my true love she were gone,I would surely find another,
Where wild mountain thyme,grows around the blooming heather,
Will you go,,,,,
You could try some 'arranging' by substituting relative minors for their respective major chords.
Along the way you'll discover some terrible combinations of chords. That's OK. It practice. And when you practice it's important to know it's OK to make mistakes.