Fulton Armory has a couple of WWII training films, in DVD format, which are interesting.
"M1 Rifle Marksmanship - Part 1" - Use of sling, controlling your breathing, squeeze the trigger, etc.
"M1 Rifle Marksmanship - Part 2" - Sight adjustment for targets at different range and allowing for wind.
Of the two, IMO, Part 1 is more useful. I'm fairly certain that present day competition doesn't use the "hasty sling" in the standing position as is shown in the films, but the other sling positions are accurately shown.
Surplusrifle.com has .pdf files that you can download that show field stripping various parts of the M1 Garand - Collecting and Shooting the Military Surplus Rifle (2005) - Surplusrifle.com
"Will shooting it affect its value?" Hard question to answer. If you shoot thousands of rounds annually, you're eventually going to wear out the barrel and replacing it with a new one will destroy the "original" status.
Some folks have "shooters", which are sort of mutts rather than "all-original" collector's items. As such, rebarreling these doesn't do much to reduce an already low value...