1. Ammo for the range - Federal top gun, Winchester AA and Remington gun club are three popular target loads. I prefer top gun in my 870.
2. Ammo for HD - 00 Buckshot is the standard, lots of variety but any decent buckshot will do just fine. I prefer Federal Vital Shok.
3. The first accessory any new shotgunner should get is a case of ammo. Shoot through that before you add anything to your new gun. This will help smooth out your new gun, and familiarize you with it. After that you will have a much better idea about what extras if any you want to add.
4. The only tool needed for complete take down is a hand punch or similar tool (like a small piece of a wood dowel) to push out the trigger pins. You might be able to push them in slightly with your finger, then grab the end that is now sticking out and pull the pin.
5. Shooting with no choke -
NOTHING should be fired through a shotgun with no choke installed. If you do, the threads will likely be marred beyond repair on the very first shot. Do not do this.
6. Foster or "Rifled" Slugs can be fired through a choke up to full. Contrary to it's name, the rifling on the outside of the slug does not stabilize it. Today, the lands simply allow for the slug to swage itself through chokes, up to about full. You will likely find however, that best accuracy will come from more open chokes like improved cylinder, or cylinder.
This a link to the Shotgun 101 library, which will tell you anything you want to know about shotguns.
http://www.thehighroad.org/showthread.php?t=464149