If your car is leaking water, you can sometimes easily find the leak DIY.  A mechanically inclined person can sometimes find it without costly professional help.  Here are some tips:

Ask the right questions

1. Has this area recently been worked on? (windshield, tail lights etc.)
2. Do I park uphill/downhill (important for duplicating the leak)
3. Does the car have a sunroof (possible clogged drains)

The Diagnosis

1. Remove any interior parts for visibility and have the interior dried (summer time is great or use a space heater)
2. Have a friend spray water and have one person inside find the leak
3. Follow the leak as far as possible and mark it.
4. Test multiple suspect areas as there is often more than one