To drain the tank, switch off the water. You can find the valve near the wall behind your toilet. Once the water is switched off, flush your toilet. This should drain all the water from the tank. Look at how dirty your tank is. If it seems relatively clean, all you need is a basic disinfectant. You can use the same spray you normally use in your bathroom. However, with built up debris, you need something stronger. If you notice hardened mineral deposits in the tank, opt for white vinegar. If there is a lot of gunk and mildew built up in the tank, clean it with bleach over a commercial cleaner. With bleach and commercial cleaners, you can spray or pour the cleaners into the tank. Target the bottom and sides of the tank, paying particular attention to areas with built up dirt. Make sure to wear gloves when handling bleach. When dealing with mineral deposits, the process is slightly different. Pour white vinegar into the tank, up to the top of the overflow tube. Let the vinegar sit for 12 hours before flushing the toilet. After 12 hours have passed, flush the toilet and proceed with regular cleaning.

Summary:
Drain the tank. Determine the proper type of cleaner. Apply your cleaner accordingly. Let vinegar sit to treat mineral deposits.