In the past, a regular 60 or 75 watt household incandescent bulb could be used to provide basking heat for turtles. However, those bulbs have been phased out and replaced with more "energy-efficient" ones that produce less heat. You'll need to find a bulb that generates enough heat to keep the temperature at your turtles' basking area around the high 80's to low 90's F.
I'd suggest that you try either a 43 watt halogen bulb (which replaces the traditional 60 watt bulb) or a 53 watt halogen bulb (which replaces the traditional 75 watt bulb).
NOTE: Spiral (compact fluorescent) light bulbs give off very little heat, so they will not serve your purpose.
After the light has been on for awhile, check the temperature at the basking area with a digital thermometer. If necessary, adjust the distance between the heat lamp and the basking dock to achieve the desired temperature.
Lights should be kept on for about 12 hours a day. If you want, you can coordinate your turtle's lighting with your area's season by keeping the lights on longer during the summer than the winter.