We are getting a ton of new players right now to DayZ, and I hope some of them are finding helpful posts on this site.
The DayZ Beginner Guide on the wiki is a great guide if you are new to both DayZ *and* ArmA. You get a rundown on the HUD and icons, how the different status effects work, also how to eat and drink. The section on gear and inventory is invaluable at first. I figured out most of it on my own and deleted some good stuff that way.
Remember: when switching guns take the gun FROM your bag and put it INTO your hand. If your pack is full and you do it the other way around you will lose that gun forever. Also be careful dropping stuff on the ground, especially roofs or floors in a building. Sometimes stuff disappears. Grabbing a new bag can also cause you to lose the old one – with everything in it.