Before I get to your specific flash question . . .
I have been photographing weddings professionally on a part-time basis for a number of years. IOW, I also have a "day job." The weddings are not the "high end" ones so often depicted in magazines. They're the mainstream middle-class ones, and I work them "solo" without an assistant.
Unless this is a *very* casual ceremony with only a few in attendance, shooting a wedding is a huge responsibility and entails a *lot* more than those who have not done it before can imagine. It will challenge everything you know about photography, and then some. It may also exhaust your diplomatic skills working with people you have never met before . . . who may or may not be all that cooperative or respectful of what you're trying to do. It's not that common, but it can happen. Paid photographers are also subjected to this, but probably not quite as often. The key is the respect and cooperation you get from bride/groom. The rest generally follow their lead.
I have a "wedding survival guide" that I wrote specifically for those who have been asked by friends/relatives to shoot their wedding:
On to your question:
TTL = "Through the Lens"
Built in exposure meters on SLR's measure the light coming through the lens. Only that portion of the scene that will end up on film influences the metering, and consequently the exposure. Although it's not 100% perfect in any camera, it is inherently more accurate than other reflected metering methods that average across an entire scene as it is not influenced by anything just outside the view of the lens.
TTL Flash Control makes use of the camera's metering to control how much light the flash puts out. To do this there must be additional electrical connections between flash and camera beyond the one used to trigger the flash. Each camera manufacturer has implemented TTL flash control differently, including the number, location and function of electrical contacts in the "hot shoe." Becase of this, TTL flash requires a "dedicated" flash unit that is made specifically for your camera brand and model.
For Nikon system TTL flash units, I strongly suggest the SB-28 or SB-28DX which you should be able to find used. It was recently replaced with the SB-80DX which is quite similar. You will NEED the flash power these are capable of producing. A bit of "overkill" in power capability allows your flash to recycle much, much faster if it's not having to punch out everything it has (it keeps what it didn't use). The recovery time difference is dramatic . . . from 6-7 seconds down to only 2-3. Might not sound like much, but 6-7 seconds waiting for flash recycle at a wedding doing the portraits, or during the major reception events seems like an eternity . . . and can cause you to miss candids.