I'm trying to hardwire my radar detector and I have no idea how to connect this to the fuse box.

Do I need an "add-a-circuit" kit for this or is there any other ways to do this?
(Sorry about the ignorance, I'm a total newb when it comes to stuff like this)

Any helps are much appreciated!

Go by your local Radio Shack and ask about "Fuse Taps". A Fuse Tape is a device that you place on an existing fuse and it provides a location to attach another wire to that circuit. They should have an assortment of different ones that work on different types of fuses and they should be able to help you find one that will work.

http://www.bussmann.com/images/1c4adfe6-eff3-4dd3-9dc6-0d727a44c382.JPG

An example.