Must haves for Nikon D3100?

Are there any must have accessories for the Nikon D3100? And also if you could Please answer this! I was looking at some other DLSR cameras and they have polorizers; can you get a polarizer for the NIkon D3100!

Polarizing filters are for the lens, not the camera and you can purchase them for any lens.
Do you know why you would want or need a polarizing filter? Because if you don't, then it's kind of useless to you.
Many people choose to use a UV filter as protection for a lens.
Must have accessories? Hmm, a lens pen for cleaning the lens is probably my biggest must have.
You will eventually want a speedlight/hotshoe/external flash
A remote
A good camera bag
Spare battery
A few books on photography:
Understanding exposure by Bryan Peterson; The Moment it Clicks by Joe McNally; Digital Photogtraphy the boxed set by Scott Kelby

None, really.

Other accessories are (non of these are must have unless you do a particular style of photography):

UV filter (multi-coated) to protect the lens. Not necessary, but nice to have in some situations.

Polarizing filter. Needs to be circularly polarized. You get a polarizing filter for your lens, not the camera body. Your lens has a thread size listed on the front of the lens. If you have the 18-55mm kit lens, the thread size is 52mm diameter. This is good for making the sky a darker blue and reducing glare. Depending on your shooting habits, you may find that you don't use it that much. I rarely use my polarizing filter, but when I need it it sure is nice to have.

Perhaps a camera bag.

In the future, you may want different lenses. Wait a few months until you figure out what you need.

Remote cable. Useful for certain types of photography, especially studio work.

Tripod. Lots of uses for a tripod, especially if you do studio work or shoot videos. Nothing worse than watching someone's hand-held video work.

White balance card if color work is critical.

If you buy the standard factory kit, everything you need is there except for the memory card. If you buy from an authorized reseller, they may provide additional "necessities" like the memory card, cleaning kit, camera bag, extra battery, etc. To justify a higher price tag. The camera, the memory card, and the user's manual are all you will ever need. Anything else is just an accessory you can live without, including that polarizer. If you don't know what it is, don't buy it.