Follow along at the link and learn how to build and add a seat to an existing deck.
At the link you will find plans to build Stairs and railings for an Outdoor Porch Deck. Scroll down the page and find PDF number 6407.
At the link you will find lots of miscellaneous building plans including one for a 14 ft x 16 ft post and beam support deck. Scroll down the page and find the PDF nubmer 6406.
Mark off the deck area using string. The string will help you visualize the size and appearance of the finished deck and will also serve as a guide for excavation and post placement. But trust me, it will always look smaller this way than when it is done.
At the link you will find lots of free information to get you on your way to building a deck of your dreams.