Keep an eye on the new cans as they're being brought in. Make sure they look new and don't have paint in the rim of the can. If it's a five-gallon bucket, check to see whether the lid is still sealed on with the plastic strip. The only time it's acceptable to mix water in the paint is when you're using a deep or ultra deep base paint to reduce its stickiness, which is rare with new paint technology. Dark primary colors are composed almost entirely of tint that makes it very hard to work with without adding water.
To get basic measurements, measure the length and width of each wall you want to paint in inches. Multiply the length and height together, and divide by 144. This will give you the square footage for each wall. A gallon of paint typically covers around 400 square feet. In some cases, you may need a primer 2 first, then a single coat of paint, meaning that you may need one gallon of primer and one gallon of paint to cover 400 square feet. However, if the wall requires multiple coats, this will increase the total amount of square footage required, and therefore the amount of paint.