You first need to determine what is missing from the equation; spark, fuel, or compression. Take the plug out, and ground it to the motor while turning it over, look for a strong blue spark. If you have spark, see how well it starts with starting fluid. If it fires easily with the fluid, you may have a fuel problem. You may just have bad gas, or it could be a blockage in the fuel system, a failing fuel pump, or bad vacuum signal line to the fuel pump. Take the line off the carb coming from the fuel pump and turn the motor over, see if any fuel comes out. If you can't find a problem with the spark or the fuel delivery, then you may need to do a compression test. I would certainly get yourself a good repair manual to start.
Yours truly, sincerelly, love always, Josh aka The DirtDemon.