Chances are, you will have your laminate top installed by a professional installer, and they will come out and do your measuring for you. In your L shape design, you would measure the long run from the back wall of the corner, then the short run from the opposite back wall of the corner, then subtract the width of the material. The material width will vary depending on the brand and color of material. (its not all 4 feet wide, though most is)
Seams are typically done where they will show the least, usually at a sink, or stove. Seams are not done well at corners, because corners have a tendency to move around, and the seam more likely to open. I have never seen a 45* seam in a corner as Wick described, though the rest of his information is good.
Because of the preference for seam locations, you may discover that you actually purchase far more in linear feet of material than you have countertop. In addition, with many patterns and styles, you must have ALL of your laminate lay the same direction, so you cannot economize by turning a scrap piece 90* to fit a corner or something. Good luck