Fishless cycling is when an aquarium is run without fish, but with a daily dose of ammonia to simulate the waste products of the fish.
The idea is to build up the beneficial bacteria in the filter that will take care of the fishwaste, and convert toxic ammonia and nitrite into much less toxic nitrate.
Use this calculator to determine how much ammonia you should add to reach the desired concentration in the tank (typically 2-4ppm is used)
The unit ppm (parts per million) is the same as mg/liter
A pharmacy usually has syringes that can be used to dose small volumes.