The lower-cost backdrop stand "kits" sold on
eBay are actually "not all that bad". However, you can also use a 9.5 foot long piece of aluminum pipe that will go inside of a roll of 9' wide Savage brand background paper, and a couple of Home Depot "J-hooks", or something similar, fastened to an overhead spot on the wall,etc. However, that's kind of a permanent/semi-permanent thing; the background stand "kits": have 2, usually 13-foot stands, and typically a three-section crossbar that assembles to be just over 9 feet long. Usually sold in a carry bag. Look around...the cheaper ones are $79, last time I looked.
The D3100 has no PC outlet on the body, so the Nikon AS-15 (Accessory Shoe, #15) allows you to hook up a 10- to 20-foor PC connector cord to most flash units, so you have a so-called "wired" connection from camera, to flash.
Umbrellas are inexpensive. I recommend the Photoflex RUT convertible umbrella in 45 inch size. For a flash...sooo many to choose from. You also will want a 9-foot light stand.
Start with a gray background paper. Think "Thunder Gray"...Google that...
Some people suggest "triggers" for tripping the flash....yeah...but for $10 + the cost of the AS-15, you will have a simple solution that will trigger the main flash each time. Optical slave trippers can then be added to additional flash units, so that they all go off at the same time.
Alternately, you can use the pop-up flash on the D3100, set in the Menu settings to MANUAL flash control, and about 1/8 Manual power output, to trip OTHER flash units which have an optical slave tripper hooked up to them.