Do you know how to use the circle of fifths to show you the chords in simple songs? Apologies if it's obvious / you already know what I'm about to say but it was a complete revelation to me when I discovered it a few months ago!
Basically the 'main' (diatonic) chords that are most likely to be in your song cluster around the key you're in on the circle. So if you're in C, the diatonic chords are the ones surrounding C on the circle - ie F, G, Am, Dm and Em. The reason those are the diatonic chords is that they are the ones that only include notes that are in the scale of your key. If you learn the chords in the song as their number in alphabetical order (ie C = I, Dm = II, Em = III, F = IV, G = V), then to transpose to a new key you just move around the circle, making whatever key you've chosen become the 'new' I, and look for the new chords that cluster around it. So in the key of G, G is now I, Am is now II, we've added a new chord - Bm - as III, C is now IV, and we have D as V (still in alphabetical order, just shifted...). They are always in the same relative positions. The important thing is learning the chord progression as numbers, not just as letters.
I'm sure there are loads of YouTube videos explaining it better than that! You can buy or make your own 'spinning' version if like me you have to turn maps around to work out which direction you're going in.
The circle of fifths is definitely the most useful music thing I've ever encountered. There are patterns in it everywhere - eg from it you can deduce all the major and minor arpeggios/chords, the pentatonic scales, the full scales etc etc. There must be loads of patterns I haven't spotted yet.
I'm still a bit mystified that I never learnt much about it when I had all those music lessons years ago - all I remember was something about how it showed the order of adding sharps and flats, and it had no practical use that I could see. But actually it holds the key (for me at least!) to understanding harmony and playing better.