What does a turtle need to be happy?

do you mean a domestic turtle or a wild turtle?

Domestic - salad and greens, (they eat vegetables and stuff) water, (duh) sand, leafy conditions (gardens are good, but they might eat your plants) and a slightly warm climate

Wild - anything they can find, such as the sea, pretty obviously, a nice sandy beach and plenty of grass and vegetables to eat and steal, though you don't really hear of stray turtles sneaking into gardens and eating vegetables...

Hope this helped - i used to know someone who owned a turtle, they told me this so dont blame me if a turtle relying on this answer is unhappy :)