A good first song: slow, four open chords, and the same line keeps coming back with only the last chord changed.
knockin on heavens door
The whole song
G, D, Am. Then G, D, C. Round and round.
Give G and D two beats each, and give the last chord four. Our chart runs at 70 beats a minute, which is slow enough to breathe.
What to practise
G to D comes up every line, so spend the most time there. Then D to Am, D to C, and Am or C back to G. Give each change one minute on its own, four beats per chord, without stopping your strumming hand.
Play through it straight-down strumming at first. When that is comfortable, try this on each chord: down, down-up. It takes a plain strum and gives it a sway.
Listen for it
Stop on Am and it feels unfinished. Stop on C and it is brighter, though it still wants to go back to G. Try playing the Am line softer and the C line a bit fuller.
You're done when: you can get through the whole song with the record, without stopping, at any speed.