First of all, you need to contain the leak and prevent any further damage. Put something to collect the water under the leak.
Next, get a screwdriver. Locate the center of where the water is coming through and puncture it in the middle with the screwdriver. It might seem strange to do this, but it is important. The new hole will allow the water to drain smoothly and relieve the pressure on the rest of the ceiling. If the water is allowed to pool, it could cause the ceiling to collapse.
The next thing to know is that a leak in the roof is not typically something that you can fix by yourself. Professional attention is needed.
When the professional comes, you should be aware of what is happening. An experienced roofing contractor will inspect the roof on both the exterior of the roof as well as inside your home.
It is important to act as quickly as possible to address the situation. Severe structural damage can result, or you may end up with a real mold problem.