The basic ingredients used to make a thunderstorm are moisture, unstable air and lift.
Moisture is needed to form the clouds and feed the thunderstorm.
When the clouds become to heavy and saturated the water vapor falls out of the cloud as rain.
Next the instability in the atmosphere allows the relatively warm air at the surface to rapidly rise, creating an updraft.
The updraft helps to feed more moisture in to the storm, allowing the thunderstorm to grow and strengthen.
Finally, you need lift which can form from fronts, sea breezes or mountains.
The upward motion of lift helps to strengthen the updraft, bringing more warm air in contact with cool water vapor in the clouds and ultimately causing thunder and lightning.
The severity of the thunderstorm depends on the difference in temperature between the air and water vapor.