There are three things you need to do-
First one is learn about the nitrogen cycle and how aquarium filtration works because you will want to establish a biological filter to keep the water safe and clean naturally. This will eliminate all smell, as well as toxic ammonia from the turtles drinking water. Also, you can just change 30-40% of the water every 3-7 days (as the turtle grows it will cause more waste) this way.
You will need a proper quality aquarium filter. For a larger turtle like you have once you have it in its adult tank it will need a large canister filter. For now you can use an internal like a tetra 30i. You also need an aquarium heater. Get a titanium one. The turtle may break glass. YOu also need UV light. Ideally, you want something like a mercury vapor bulb. It's UVB/UVA as well as heat. That's everything you need. And water conditioner. (Seachem Prime is best).
So what you're going to want to do is go to an aquarium store, buy something called cycle, or Bio Spira, or Stress Zyme, they're all similar, and follow the instructions on the bottle for your turtle tank. You should also buy a good filter and heater and other equipment. The turtle needs properly filtered water, the ability to change its temperature, and UV light to be happy. Also, they can get a foot and larger. You will need either an indoor pond (ideal). Doesn't need to be more than 10" deep or so. OR, a large aquarium at least five feet by 2 feet, ideally. They really get big. There are smaller turtles that stay five or six inches but this is not one of them.
If you don't follow proper filtration the turtle will not live as long and if you don't provide UV it will not grow large and it's shell will develop a sort of pyramid malformity and the turtle will not be healthy.
YouTube - Pet Turtle Care : Mini Pet Turtles
YouTube - Pet Turtle Care : How to Take Care of Turtles