Hi wendilla
A spacecraft in low-Earth orbit is travelling at a speed of 17600 MPH and takes 90 minutes to complete one orbit of the Earth. To achieve re-entry with the minimum expenditure of fuel, the craft will fire its rocket engines against the direction of travel and slow down to a speed of approx 17000 MPH. This will result in it entering an elliptical orbit, losing height and striking the upper atmosphere on the opposite side of the Earth.
Due the atmospheric drag, the ISS loses a few kilometres of height every month and has to have periodic re-boosts from an attached rocket.