You need to go to the digitutor I provided above. And then go shoot about a thousand frames and edit these in the computer.
Here is the way to do this:
First, turn on the camera and hold down the "qual" button and the one that has the =/- on it to the right of the release button.
Set ISO to 400, auto ISO OFF "P" mode
You are now in AF-S and single point focus in the viewfinder. Now, step outside and shoot some cars moving toward you. About ten shots.
Now, on the left lower side of the lens is AF Mode button. While pressing this, rotate the main control dial, the one your thumb falls on at the back of the camera at the top right, and viewing through the viewfinder you will see a letter on the bottom change from S to C. Leave it at C and go shoot the same ten shots of cars moving toward you.
Now shoot ten more shots, but about three feet from a large tree, focus on the trunk, and while holding the release button halfway down reframe the photo which has the tree on one side, then push the button the rest of the way down and take the exposure. Do about ten of these.
OK, back to the AF MODE button...press and while holding this rotate the main control dial again to show the "S" in the viewfinder.
Then go shoot the tree shots again.
Then, load all forty shots into your computer. Examine each one and see what you have.
The first ten, some will have been on focus, some not , some the camera will not have even fired and you will think it is broken.
Second ten...most should be perfect.
Third set of ten...mostly out of focus tree......
Fourth set tree in focus at side of frame.
You now know everything there is to know. Unless you want to read the manual. But with the information above you should understand what the things do. Let us know if you are able to do this and get the results.
As far as shooting a wedding...because of the high stress level, the critical timing, the need to have the camera function as a second arm, without even thinking about it, I would highly suggest you not do this until you have at a minimum six months or about 10,000 - 15,000 clicks on your D800. And have processed all of these in your computer yourself.