What CTyler is explaining is how to use the 3,4,5 triangle to find a right angle (also called the Pythagorean Theorum). This theorum says that when you square each of the 2 right angle sides (legs) and add them together, they always equal the square of the third side (hypotenuese) of the triangle. Mathematically this is written as A squared + B Squared = C squared. It is called the 3,4,5 triangle since the smallest size whole number triangle this works for is one that measures 3 by 4 by 5. (9+16=25). While CTyler has the instructions correct, his numbers are off. You should use 30,40,50, not 20,30,40. If you follow his directions, but where he said 20, use 30, where he said 30, use 40 and where he said 40, use 50, it will work. Now, 50 feet is pretty big, so a better idea is to use some other multiple of the 3,4,5 triangle, since 3,4,5 is too short to get your longer sides straight. If you are building a 20x30 building, use 12, 16 and 20 for the measurements, and it will come out the same, since 12,16,20 is the 3,4,5 triangle multiplied by 4.
As for the rest of your planning, if you don't have someone who has done it a few times before actually on the job site, you will need to get a book on Pole Barns. Take a trip to Lowes or Home Depot or Barnes and Noble and buy one. I have the one by Monte Burch, but there are others out there as well. Even if someone is helping who does have experience, you will be much more useful on the job site if you have read one of those books through, in my opinion. With one of those books anyone who can read can do it if they have time, energy, money and a few good friends willing to help! Good Luck!