How to use a nikon speedlight on a Nikon D3100 without extra components?

I have the Nikon D3100 and want to rent first, then buy (when I have enough saved up) a nikon flash. Problem is, I heard something about the d3100 not having something inside the camera to use nikon flashes… I don't remember what It is but I think it has to do with flash. And that I would have to buy some kind of cord or transmitter, or smething.

Question is, How can I use lets say the Nikon SB-800 or 900 without having to buy another $100 accessory? Does anyone know of any Nikon flashes that will successfully work with my Nikon D3100 without me having to by another cord, or something that will cost another $100?

I heard the SB-700 will work with my d3100 without anything else but I want to hear from people who know both my camera and lens well.

any help would be greatly appreciated!

They all plug into the hot shoe - that's all you need.

It's nice to have an off-camera lead, but red-eye reduction tends to render this unnecessary

You heard wrong. The flash will slide into the hot show on top of the camera. The camera may have a piece of plastic you need to remove first.

The built in flash on the d3100 does not have commander capability to operate off camera flashguns.
You will need to put a CLS capable flash on the hotshoe, this can then act as a commander. I think the sb700 is compatible.

The flash will work OK on your camera. What your camera will not do is control the flash in wireless remote mode - with the flash not connected to the camera. This is what you are probably thinking of. You indeed need either a cord or SU-800 transmitter (that connects to the hot shoe) to remote control the flash.

So if you use the flash on-camera, you will not have any issues. If you want to remote control the flash, you will have to buy a SU-800 or cable.