RTFM (read that fine manual) first. The owner's manual that is.
Youll find instructions for field stripping your new gun, and directions to clean it before shooting it to remove the factory preservative, which can get gummy/sticky/adhesive- especially in the chamber.
With the 7-shot version, you'll have to remove the magazine extension before you can take off the barrel. First remove the cross screw in the clamp with a properly fitted screwdriver, and take off the clamp, pulling it open a bit as you slide it forward off the barrel and magazine extension.
Then unscrew the magazine extension tube. Remington's factory extensions are two- piece constructions, so don't be concerned when the tube unscrews and the magazine tube coupling doesn't. NOTE- the tube contains the magazine spring and will seek to escape when you completely unscrew it. Be sure to contain it. Then remove the magazine spring too.
Lately Remington seems to have been "gorilla gripping" the magazine tube coupling at the factory- they can be awfully difficult to get loose the first time, according to other reports seen here. It may be you will need a pair of slip joint pliers or the like, padded with leather or several layers of terry cloth to avoid damaging the coupling, to get it loose. When you get the coupling off, then you can proceed with field stripping.
It might be helpful to have someone on hand who knows 870s well to help if necessary, and offer aid and comfort if needed. If you don't know anyone you can ask for such help, proceed as far as you are comfortable on your own. I'd go at least as far as taking off the barrel to give it a good cleaning.
I've found WD-40 to be a useful degreaser, other solvents work well for that too. Remington suggests soaking the exterior down in RemOil and letting it sit for a while before wiping it down. Pay particular attention to the chamber and the bore and make sure all the factory gunk is out of there. Failure to do so can lead to sticky extraction.
Some brands of shells seem worse than others at sticking in the chamber when fired- Winchester's 'loss leader' bulk packs seem to be pretty common offenders at that, but other brands have problems sometimes also. Best bet is to buy just a single box each of various brands/loads till you find out what your gun likes. Then buy larger quantities.
Start with light birdshot field loads to begin with, you need to learn the gun without getting pounded by unnecessary amounts of recoil. There are some companies offering loads with 7/8 ounces of shot, and those are soft shooters if you can find them. The lighter the shot load in ounces, and the lower the velocity (or the smaller the powder charge in drams equivalent), the lighter the load will be.
Read Dr. Dave's 101 threads here (
http://www.thehighroad.org/showthread.php?t=2766 ) for pointers in getting started, and/or find ourself a mentor to show you the ropes. Good instruction cuts learning curves sharply and helps avoid picking up bad habits that need to be unlearned later.
Have fun and Stay Safe,
lpl/nc