If it is a Red Earred Slider or something similar get ready to shed some dollars on a set-up. I've have my slider for ten years. Today I just installed her new filter after my other filter died....$350 bucks

Sliders can reach 12+ inches and require lots of space. I keep mine in a 120 gallons aquarium(which is still to small). Tank, stand, lighting, basking area, ran me about $2000.
My torts are way easier, I just converted a spared bed room into a jungle($500-$700)

Turtles are cuties, but in my experience not the easiest to keep.