i'm not really sure how many gallons it should be, and i think it depends on the type of turtle
for the environment, you will need to partially fill it up with water (depending on your turtle's size) have a rock or log for the turtle to lay on, and a light so the turtle can bask. you will probably want some sort of filter also, because turtle tanks can get dirty very quickly. and be careful when you buy the turtle, because a lot of times the turtles at the pet store have diseases. avoid purchasing a turtle with white specks/clumps on its shell