This is what I have
Tank
3 of my sisters are living happily in a 30 gallon aquarium. The rule is 10 gallons per inch of turtle but you could get away with less than that. I have about 7 inches in the 30 gallon and they are happy.
Filter
Turtles need clean water otherwise they will develop shell rot
.I have an emperor penguin biowheel 330 on the tank and only run it during the day (It is noisy). You could probably run another filter in there and it doesn't have to be expensive because turtle's aren't as picky about water conditions. I chose the biowheel because it has a long intake without a lot of sections to suck air.
Basking
Turtles need to bask to regulate body temperature and to help in the digestion. You can buy basking lights at pretty much any pet store. You can either hang them from the ceiling, clamp it to the side of your tank, or let it rest on the screen hood.
Turtles need UVA/UVB light for their shell to be healthy. This can either be a floresent light or a basking light. It will say on the box if it has UVA/UVB lighting.
The basking area is a solid area (I don't think plants would work but I am not sure). The area needs to be able to absorb heat to heat the turtle evenly, no plexiglass unless you glue some pebbles or some cork bark on it. The area needs to be so the turtle can get on it fairly easily and the turtle is fully out of the water when it is on it.
Water Level
My tank is about 2/3 full of water. I think this gives them plenty of swimming space.
Hood
I have a screen hood. Some come with hinges so half of it opens which is very nifty. No plastic otherwise it will melt under the basking light.
Water temp
I recommend having a water heater in the turtle aquarium, get an unbreakable fully submersible one since they will almost always break the glass ones. Check where the turtle you are getting is from and set the temp accordingly.
Plants
You should have plastic plants that float or reach the surface so the turtles can sleep on them. DO NOT have real plants as the turtles will destroy them.
Food mmmm.
I feed my sister's turtles Reptomin as a staple and crickets, worms, etc. They will eat pretty much any insect except ladybugs. You can also give some beef heart to them as well.
As for vegetables, they like Anacharis, romane lettuce, carrots(helps rid of worms) just to name a few.
This is a link of a great site for turtles and forums.
turtletimes.com
Hope I could be of help!
Don
Here is a pic of my sister's setup.