G major is a friendly guitar key and a common folk key. The tonic chord is G. The neighbours you will meet first are C and D.
Amazing Grace is written there. The chart uses G, C, and D. That is I, IV, and V in G. Once those three sit in the hand, a large corner of the Songbook opens.
The key page lists the diatonic chords and the public-domain songs stored in that tonic. Use it as a shelf, then jump into a page and play.
In G major the major triads are G, C, and D. The minor triads are Am, Bm, and Em. The diminished chord on the seventh step is F#dim. Most singable pages skip that last one.
Em is the relative minor. The E minor page sits next to G on the circle. Songs that feel sad in G often lean on Em for a line, then come home.
C major is a fourth below G. D major is a fifth above. Those two keys share enough shapes that a capo or a transpose chip can walk you between them.
On a song page the key is a query, not a new address. Amazing Grace stays /songs/amazing-grace/ whether you play it in G or in C. The chart and the pad move together.
If G sits too high in your voice, try C. If it sits too low, try A or D. Listen for the highest note you actually sing, not the name of the key.
Ode to Joy and When the Saints Go Marching In are useful after Grace. They keep a small set of letters so your hand is not learning a new family and a new melody at once.
The Four Chords in G is G, D, Em, C. That is the wheel with a G home. Three-chord Song in G is G, C, D. That matches many hymn and folk pages.
A twelve-bar such as Twelve-bar Blues in G is a longer story. Learn the three-chord hymn first. Add the blues walk when the three shapes are automatic.
The chord player online is the same instrument. Open it, pick a G-major page, and stay there for a week if you like. One key is enough for a first month.
Borrowed chords show up on some pages. Treat them as guests. If a chart suddenly shows an F in a G song, it is colour, not a new home key.