How would you fasten the double 2x8's to the concrete footings? Do they make special brackets for this?
How many concrete footings would you have on an 18x14 deck?
It seems pretty obvious at this point that you are in need of much more knowledge then can be given here. You need to get a few deck building books and gain a basic knowledge of deck structure and terminology. Once you digest those, come back and fill in any gaps and clarify missed points or confusions in the books. You can also get videos that can help alot at your stage.
You want to understand what your doing and why your doing it so the structure is safe when it's done.