If it's a brand new tank you really don't need to disinfect it. Tap water should contain enough chlorine to kill anything that may be airborn. Use a good clean paper towel to wipe it down, and then fill & begin the cycle.
If it's a used tank, 1 part bleach to 20 parts water, wipe it down with that. Rinse with tap water on a paper towel, and let it dry.
If you have your own well, 2 drops of bleach per gallon of water approximates what municipal water supplies put in tap water. it's an old camping/survivalist method of making water from a questionable source drinkable.