All you need to know about off camera flash with speedlights is here...
http://strobist.blogspot.co.uk/2006/...hting-101.html
You can take awesome portraits with natural light, you just need to know how to use it to your advantage.
If it is OCF you are interested in......the the strobist link I posted will help. It can be real confusing at first.
Your camera doesnt have nikons Creative lighting system, which is a system that allows you to control groups of flashes off camera.
So another option is to use a trigger and recievers to fire the flash/flashes.
These can be picked up online for not much money........ yongnuo, phottix, and pocket wizards are such brands, yongnuo being the cheapest and pocket wizards being the more expensive.
Hope this helps
Lee