Roman Numerals - sounds tricky? not really! Makes everything easier
So you're playing Cuan Grande es Mi Dios (How Great is Our God) in G major. What's chord 1? G right? Simple. Chord 5 is D, 4 is C. They are the most common. There are others,
Am is chord 2 - and it's minor
Bm chord 3 - minor
Bb chord 4 - major
C chord 5 - major
Em chord 6 - minor
So if you play a song that is in G it could look like:
G - D -C - Em - C
It could be written I - V - IV - vi - IV
So if you want to play the song in D you just play chord I of D, then chord V - IV - vi and you've changed key but the chord number is the same.
That's it. Once you know the names of the chords in a key you can work out the number, memorise the number and transfer to other keys more easily. Try playing a these chords: G - D - C - Em - C and change the key to D then F. Here's what you should get (don't see this till you've tried it!):
In G you'll get: G - D - C - Em - Bb
In G you'll get : G - D - C - Em - C
In F you'll get: F - C - Bb - Am - Bb
Oh and I almost forgot! there's a 7th chord, 7th diminished. But I can explain that one to you personally as no one really uses it.