Most of us remove he lids. Some people use lids and devise ways to be sure he turtles get the right light - like cutting a hole in the lid or only covering half the tank with a lid. Here's a link with pictures of some nice tank setups that might give you some ideas. Also, browse through the photo section and indoor habitat section of the forum as there are a lot more photos of good setups there.
http://www.redearslider.com/habitat_gallery.html
For an aquarium less than 55 gallons in size, you're going to have to have a two light bulb setup. Both bulbs will need to be positioned over the basking area. Here's a description of the two bulbs:
(1) One bulb is for heat. It's purpose is to motivate the turtle to bask and to aid digestion. You don't really have to have an expensive fancy bulb for this one. A regular household incandescent will do. I think a lot of people use a 75 watt bulb.
(2) The other bulb is for UVB. Any bulb that provides UVB will also provide UVA, so you'll have both covered. This bulb will be a fluorescent type. There are two types to choose from - tube and compact. I think most people are using a compact fluorescent these days. In the presence of UVB a turtle's body makes vitamin D which is required for the metabolism of calcium. Turtles need a lot of calcium for the health of their bones and shells and it's vital that they get good UVB. A good brand of UVB bulb is the Exo Terra Repti Glo 5.0 26 watt compact fluorescent bulb. Be sure the packaging for whatever bulb you get specifies that the bulb provides UVB and the percentage is at least 5.0. Any less than that is too little to be very helpful.