Watching and reading or good, but experience is what ties it all together. Really need to go out and practice and play, then go read up, or watch the videos, that pertain to the areas you are interesting in understanding better, the stuff that confuses, or just seems like there must be a better way. Just keep in mind, that there isn't always good stuff where you hunt, but you'll learn what various trash items look like to your machine. There is also some chance you might be hunting clean ground, where someone had brought in a lot of fill dirt.
For the most part, it really isn't too hard to turn on the detector, and find some stuff to dig up. I'd dig everything to start, gives you some practice on digging a plug, recovering targets, and then neatly filling it back in. You really want to get in the habit of not leaving a mess behind you. Your own property is good practice, since you can fix it, or wait until it fixes itself. My front yard seem to kind of liked me digging it up, we've had some good rain, and it's looking fuller and thicker. Could also have some to do with me getting a mulching mower last year, which also worked out well on the leaves...