Congrats! Try this as a primer...
http://www.kenrockwell.com/nikon/d31...uide/index.htm
Not a huge Ken fan, but free information is good sometimes. ByThom doesn't seem to have anything on the D3100 yet, but he's one of the better Nikon resources out there...
http://bythom.com/nikon.htm
I have a flickr pro account. Free accounts have a 200 image limit. The UI could use some help, but it's a good site for hosting and sharing.
If you really want to learn, carry a small notebook and take notes on all your photos. Take the same picture several times and vary the settings so when you get home you can see what you think. You do have extensive EXIF data, but having some notes about how you setup the shot may be helpful too.
IME, "A" mode turns your spendy DSLR into a crappy PNS camera. "P" mode gets you better exposure and flash performance... at least on my D80.