Ok - gonna try. You're gonna have to follow along carefully.
Chords are formed from the notes in a scale. If you can figure out the scale, you can figure out any chord and all of the chords in a particular "key".
In the key of "G" - you have a "G-major scale".
The notes in a G-major Scale (Do-Re-Mi) are
G-A-B-C-D-E-F#-G - or -
1- 2- 3- 4- 5- 6- 7- 8
The simplest chord is a "triad" - three notes. Root -Third - Fifth Remember that. 1-3-5 or Root-Third-Fifth. There are FOUR basic kinds of triads Major, Minor, Augmented, Diminished.
Major chord triads have a MAJOR third (two full tones - G to B) and a PERFECT Fifth G to D
Minor chord triads have a MINOR third (1 1-1/2 tones.. G to Bb) and a PERFECT Fifth G to D
G MAJOR (G) = G-B-D
G Minor (Gm) = G-Bb-D (G, B-flat, D)
If you wanted a "D major Triad" - it would be D-F#-A Root-Third-Fifth
If you wanted an A-major Triad - it would be A-C#-E Root -Third Fifth
AUGMENTED Triads have a MAJOR Third and an AUGMENTED Fifth G-B-D#
DIMINISHED Triads have a Minor Third and a Diminished Fifth G-Bb-Db
All chords can also have different "voicings" - that is , a different note on the bottom, and the notes in different order.
GBD is a "G major Triad" - the most basic G chord. In music theory it would be called G-root (no inversion) because the G is on the bottom.
Next you could have BDG -- That's the "first inversion" of the triad -- the third on the bottom .
DGB is the "second inversion" with the fifth on the bottom
They're al G-major chords, just different inversions
Chords don't have to have only three notes in them. Guitars have 6 strings, and you can use any or all of them to make chords.
On the guitar , the "typical" open G chord you learn is GBDGBG , using every string.
First finger on low G - Your second finger is on the "B" and your third is on the high "G" Some people play it with their third finger on the low G and pinky on the high G, because that makes it easier to go up to the "C" chord. - but either way is fine.
Another voicing is GBDGDG... There's still a major third there (low B) so it's still a G major chord
Go back up to the G-major scale. The other major chords you can get out of it are
C major C-E-G and
D major D-F#-A
So the major chords in the key of G are
G - the I chord (roman numeral I... the first step of the scale)
C - the IV chord... starts on the 4th step of the scale -- and
D - the V chord ... starts on the 5th step of the scale.
And you'd also have a couple of minor chords
Em - the "Minor vi " (sixth step of the scale E-G-B (minor third E-B .. perfect fifth E-G) - and
Am - the "Minor ii" (second step of the scale) A-C-E (minor third A-C... perfect fifth A-E)
So now you know most of the basic chords in the key of G.. just work the triad up the scale. Most songs go I-IV-V- I (G-C-D-G) or I-vi-IV-I ( G-Em-C-D-G) or ii -V- I ( Am - D- G) . Those basic chord progressions cover thousands of songs.
So what if you wanted to figure out the major and minor chords in the key of C ? Here's the major scale.
C-D-E-F-G-A-B-C
Now you figure out the I, IV, V chords . I'll give you a hint on the I chord.. it's C-E-G
Hope that helps