How to use Flash Triggers for Nikon D3100?

I know I will need a trigger to use the flash as a slave/wirelessly.

Please tell me which technology will it be using TTL or CTS. I'm not pro enough to use manual settings.

Will it be automatic like the popup flash (setting will be done on the camera) rather the flash trigger?

To do what you want requires very expensive flash triggers. All the cheap ones are flash only, they do not let the camera set the flash exposure for you. Pocket Wizard makes a setup that will let you use a Nikon CLS flash over a radio slave trigger system, but it costs as much as a Nikon CLS flash.

You will find that it isn't difficult to learn how to use cheap flashes and flash triggers to get good lighting. Don't let manual mode on the camera and flashes scare you away.

The D3100 has no provision to wirelessly control a flash. If you use a standard flash trigger - such as the Pocketwizards or Yongnuo RF-603s, realize they are slave mode only - so you must use the flash and camera in manual mode, and figure out the exposure.

If you want to use iTTL and the CLS system, you must buy either a SU800 - which is the part Nikon left off your camera, or the advanced iTTL capable flash triggers.(such as the PocketWizard FlexTT5 and MiniTTi). The SU800 requires a flash to be capable with iTTL/CLS, such as Nikon's speedlights above the SB400 level (i.e. SB600, SB700, SB800, SB900, SB910)

Also Debao, a Chinese marketer - makes a knock off version of the SU800 (they even call it a SU800), which is about half the price of the Nikon SU800 which might work - although I have not tried it.