Aquarium-Terrarium
Turtles like to have their own space. If you have one turtle, a 30 gallon aqaurium will suffice (If the turtle is about 7 inches long). For each additional turtle you will need 30 more gallons. This can get very expensive, so I recomend keeping the turtles outside if you have a large population (unless you live in a cold part of the world, then you might just have a "turtle room" or a room of your house with turtles.
You will need to put water that is at least as deep as the biggest turtle's shell is tall. You will also need to make the water space big enough to let the turtles swim freely. Always try to keep the water as clean as possible with a filter, but it will also need an occasional change.
For Basking, the turtle will need a space that is right above water-level, maybe a log or a small island. You will need a basking lamp that is available in most pet-stores.
The turtle will also need a hiding place where it can feel secure. I prefer having one in their water (a small cave) and one on land (A log with a hole in it or rocks positioned as a cave). Without the hiding space, your turtle will feel unsafe and insecure.
Habitation is an option. If you want the turtle to select a plant and just chomp it down, then it is a good idea. But if you are going to feed it lettuce and vegetables from the fridge, It would be more of a decoration.
The turtle will need a climbing spot for excercise. Just a few rocks, and maybe a small hill or two.
Sources:
Turtle Care Book