My turtle and i learned of the importance of basking and good light the hard way.
Turtles have to be able to get out of the water and dry off completely - top and bottom - at frequent internals. Turtles are naturally attracted to warmth. If there's a basking platform of some sort in the tank with a light bulb over it to warm the area, they will come out of the water and bask and dry out. Without drying out, their shells and skin will develop infections and fungus. The key to motivating a turtle to bask is the warmth - the basking area needs to be about 10 degrees fahrenheit (or 7 degrees celcius) to motivate them to bask.
The other very important thing about basking is that they need to bask under a good source of UVB light. Natural sunlight includes UVB rays or you can buy special light bulbs for basking reptiles that provides UVB. When turtles bask under UVB light their bodies produce vitamin D which is essential for their absorption of calcium. If they don't absorb the calcium that they eat their shells and bones will get soft and deformed, a condition called metabolic bone disease, which is eventually fatal.
Whether or not you are able to provide a UVB producing light bulb, you should still provide some way for your turtle to dry off every day. Lots of people build their own basking platforms for their turtles. Here are some guidelines:
http://www.redearslider.com/custom_platforms.html
You can also do a search in the Indoor Habitat section of the forum and see lots of examples of basking areas that people have built.
If you can't get the special UVB light bulb, you can take your turtle outside to get some natural sun periodically. In addition, there are some vitamin supplements for reptiles that provide vitamin D. Although the artificial vitamin supplement isn't as good as natural sun or a good UVB light bulb, if it's supplemented with periodic natural sun it will certainly help.
I hope that's clear. If it's not, let me know.