If you can play a major scale, start there. Doesn't matter which one. The first note of that scale will have many names. You will hear it called the root, the tonic, the I, but its the same thing. Play that note. Then play that major chord.
Play the scale to the fourth note. It will also have many names. You may hear it called the sub-dominant or the IV. Play that note. Then play that major chord.
Play the scale to the fifth note. It will have many names too, like dominant, or the V. Play that note, then likewise play that major chord.
You have just figured out how to play a rudimentary version of most of what you hear on the radio. If you can figure out what key it's in by fiddling on the fretboard, you can probably figure the rest of the chords out without too much trouble.
If you can't play a major scale, check in with the stickied posts in this forum for some guidance.
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