With a couple of stakes in some string, you can create a straight line. In order to be sure that the line is parallel to the wall, you can measure the distance from the string to the wall in two places. To make sure that everything is square we can use a little geometry and make use of a 3-4-5 triangle. In a 90-degree triangle, the square of the hypotenuse is equal to the sum of the square of the other two sides (9+16=25). This geometry also works in multiples. Therefore instead of a 3-4-5 triangle, we can multiply everything by 4 and get a 12-16-20 triangle. In other words, your 12 by 16 foot deck will be square when the diagonal measurement from corner to corner is 20 feet. With respect to concrete, our best advice would be to follow the instructions on the bag, to the letter. When it comes time to install the deck boards, be sure to install them with the crown up to prevent puddles and rot. Also, don't drive the nails or screws too deeply into the wood.