I don't believe in mitering countertops in corners, because they aren't strong enough. I believe the strongest way to do it is to place two layers of substrate, placing a seam on the right side of the corner on the bottom and the left side on the top, so that the two seams overlap. The laminate should seam away from the corners (in my opinion), ideally at the sink or cooktop, or configure your kitchen so that no seams in the laminate are necessary at all.
Unfortunately, by the nature of your question, it sounds like you are attempting to install a preformed countertop, with a built in backsplash and selfedge. The laminate for this surface is of a lower quality (thinner) so it will bend around the radiuses of the self edge, the inside radius going into the backsplash, and the outside radius at the top of the splash. This means that it will wear through the color layer sooner than a normal laminate will, it will scratch easier, and generally not hold up as well to normal use.
In order to obtain the correct miter angle, place a piece of board (call it "blue") against the right wall and into the corner, and secure it so it won't move. Take another piece of wood (call it "red") with a defined angle cut into it, like a miter. It doesn't HAVE to be 45*, but this works well. Take red and draw a line along the miter face onto blue. Now flip red over and put the flat side against the left wall so that the tip of the miter cut is still in the same corner. The miter face should be roughly near the first line you drew, but at a slightly different angle. Draw a second line along the miter face onto blue, so that you have two lines at different angles that converge in the corner.
Now you need to bisect the angle. Go find a compass (the metal thing with a point on one leg and a pencil on the other. Measure out from the corner about 16" on each line and make a mark. From that 16" mark, draw a circle about 8" in diameter, or big enough to at least cross the imaginary line between your two lines, twice, as far apart as possible. (the circle you draw should be fairly large. The larger the better.
Now, draw the EXACT same size circle from the 16" mark on the other leg, so that the two circles intersect. This will look like the Mastercard Symbol.
At the two points the circles intersect are two points on your real miter angle. Draw a third line from the corner that intersects BOTH of these points. Congratulations, you have bisected an angle!
This line is the angle of your miter cut. Pull blue off of the cabinets and cut it along this line. You now have a template to measure and cut your countertop.
I have told you this with the assumption that your walls don't have any wows or bows in them. They always do, so this angle is not going to be perfect. You can use caulk to fill in any gaps.
Make sure you cut your miters so that the counter is a little too long on each leg. That way if you need to adjust your miters by re cutting them, you will have some fudging room. Once the miter cuts are right, trim the counters to the right length.
Good luck to you. I do need to say that if you are asking this question, you probably don't have the skills to make this come out right. You might try to find someone who has a little more experience to give you a hand.