Car that has very good mileage, under 6k

I'm a teen and looking to buy a really good car, doesn't have to be sporty, or a V8 engine, but not looking like a PT cruiser or a mom car. (I live in Lithuania) Can be diesel and gas, not electric (we don't have much electric charging stations) I need a really good mileage, being a student and the money is as always very bad, so I need to save money of course. Thanks