"..if I could somehow select just me and the guitar and cut out the rest of the clip.."
You're talking about 'green-screening' (..used to be blue-screening..) to cut out your surroundings: it needs a bit of forethought, i.e; don't try to remove the background
after you've shot the material; dispense with the background, instead,
when you shoot the material.
(..Though, of course, the simplest thing would be to mount the camera low, with a view of the clouds, and for you to just sit or stand between the camera and the clouds!..)
You'll need a 'green-screen' iMovie plug-in, and then you'll need to stand, or sit, in front of a solid green (or whatever colour you choose) background. The plug-in effect will then
remove everything of that solid colour, and will let you insert into that removed area your clip of moving clouds.
Here's the
Stupendous Software greenscreen plug-in but I don't think it's a 'Universal' plug-in, so it may force your Intel Mac iMovie to run a bit slower in 'Rosetta' mode, in order for this to work.
As for making yourself semi-transparent ..that's a bit more difficult..
(..Ooops, sorry; Bengt says it's simple..!)