"The turtle doesn't grow big enough to fit even a guppie in it's mouth. But I guess the Lamps and other needed items would say the turtle is a no-go." --Mantis
And what species of turtle do you have again? There is no turtle in the world that doesn't stay that small. Even the smallest turtle is a good 6" diameter across the shell.
If it was one of those illegal turtles, then the species you have is most likely a Red-Eared Slider. These monsters get to be a good 12" across the shell when fully grown, and are by nature aquatic, preying on fish and occassionally vegetation. Look on the side of your turtle's neck or head. You should see a thick red line on either side. If you don't have those thick red lines, and instead have some yellow dots, that's most likely a Painted Turtle. Painted turtles don't get as large, only about 8"-10" all told, but also eat fish and vegetation.
Turtles are cold blooded creatures, and need to regulate their body temperature by moving around their environment, and climbing into and out of the water. The ideal turtle tank should be half land, half water, with the water being the depth of twice the turtle's length. The overall lenght/width of the tank should be large enough for the animal to turn around in. Over the land part should be placed some basking bulbs so the temp gets a good 82-86oFoF during the day, and doesn't drop much below 76oF at night. The water should be moderately warm, probably a stable 74-76oF. A UVB bulb should also be placed on a strip above the cage not more than 6" from the surface of the land/water. UVB cannot pass though glass or plastic, so the tank must have a screen top. This top -must be secure-. Turtles are strong enough to push off screen tops and climb out of their enclosures.
They can be made to eat a staple diet of turtle pellets, but they should be given some form of meat (usually live feeder goldfish) at least every other week, and veggies once a week. Turtles eat -alot- and therefore excrete -alot- so constant water changes are necessary. A fish tank filter will not support a turtle.
I hope that helps you see what you are getting yourself into.
~~Colesea