First, and most importantly,
always, always, always wash your hands after handling the turtle or touching the water. They
can carry Salmonella and you don't want to get that.
Second is they need land to sit on, or a log, anything above the water line. While it might be an aquatic turtle (Terrapin) it still needs land to come onto. If it's not, then some of this won't apply.
Third is they do need clean, dechlorinated water with filtration. They aren't fish but they still need some of the same requirements.
Fourth is they will escape their tank or terrarium if possible so make sure to have a secure lid or the water level lower than 3 quarters of the height.
Finally, feed them a mix of foods, some vegetable matter, the turtle sticks and occasionally live food.
A 10 gallon will do fine for a year or two but eventually they grow to something like 6 or 10 inches depending on the species though most likely it's a Red Eared Slider, Map or Box. I think they'll live fine at adult size in a 20 gallon long tank but not positive.
I am by no means an expert on them or kept them but I do know the basics.
