How to use Nikon TTL function on off camera mode with Speedlights?

I just wanted a Speedlight flash(External flash) for working ttl and off camera for outdoor portrait
I have Nikon D90
and I've chosen Sb-700 speedlight, any other cheaper choice?
Could i use the ttl function on off camera with just a flash and camera?
Or what other stuff do i need to buy:
E-ttl cord(nikon Ettl cord), radio flash trigger(RF-602)

TTL suggests to me that you need something connected to your cameras hot-shoe with contact that match the mount's requirements. Have a look on the Nikon website, to see if they have a cable with male connection to slide onto the hotshoe mount, and a female connector that will allow you to mount your flash unit onto it. Sounds like the E-TTL cord is what you need for starters. Not being a Nikon owner, I can't help you any further.

Nikon uses iTTL, Canon uses eTTL. Just nomenclature differences, but the two are not compatible.

If you use a SB600 or above, you can use the Nikon iTTL function wirelessly with your D90. You don't have to buy anything else, as your D90 has all of the functionality required to wirelessly control a speedlight.

I have used my D90 wirelessly with a SB600, SB700, and SB-R200 speedlights with no issues and without having to buy anything else. In fact, I can use all three together as the D90 can control multiple flashes.

All you have to do is to put your D90 into remote commander mode, put the SB700 into remote receiver mode, match the Channel and Group settings between the two, and it will work.

But if for some reason you can't use the wireless function, or you have a Speedlight (such as a SB400) that will not work wirelessly, you can use a SB-28 or SB-29 remote flash cord. These are about 10ft in length, so you are still going to be close.

If you optionally want to use RF remotes, whether they be RF-602, RF-603, or Pocket Wizards, realize that most of those do NOT do iTTL, but just slave. That means other than the flash command, there's no exposure information transferred between the camera and flash. That means you have to put the camera into manual exposure mode and calculate the exposure.

PocketWizard does make a version that will do iTTL, but they are very expensive, and cost more than a SB910, so you might as well buy the Nikon Speedlight.