You will need 1 cup of household bleach (5.25%) for every 50 feet of 6-inch diameter well depth, or about 6 cups for a shallow well (3 feet in diameter).
Mix the bleach with a gallon or so of water and pour down your well, being sure to wet the sides of the casing. Let the solution sit for 1-2 hours to mix.
After the 1-2 hours, open each faucet, including showers and out-door spigots, until you smell bleach. Then turn the faucets off and let the bleach water sit in the pipes overnight. If your system is equipped with a filter or treatment system, be sure it is not bypassed.
The next day, open all faucets and run water at each tap for several minutes, and run outside spigot until you cannot smell bleach (running the outside spigot protects the septic system).
Have your well re-tested in approximately 1-2 weeks.
Consider having a water-tight well cap installed to reduce potential for bacterial contamination in the future. These caps are available from well drillers and are a good investment.
Seriously consider having your well casing raised above the ground (eliminating the well pit) if your well casing is buried below the ground. Well pits represent a significant risk of contamination of the well water.