How does a spacecraft get to where its going?
In order to leave orbit, a spacecraft needs to be going fast enough to break free of gravity. A huge push is needed to do that. Either that push was given to a ship as it was launched or it is given to a ship already in orbit. To push a ship that is already in orbit farther from the planet, thrusters should be fired at the highest point in the orbit. Generally, a ship will go into higher and higher orbits until it intersects with its destination.
Spacecraft can go from planet to planet that way. Even if a ship from the Earth leaves Earth orbit, it is still in orbit around the Sun. Huge amounts of energy are needed to push a ship fast enough to break free from the Sun's gravitational pull.
To leave orbit and travel towards the body it is orbiting, a ship only needs to slow down (by retroburning or aerobraking) and wait for gravity to pull the ship in.
How do we put a spacecraft into orbit?
Once a ship is in orbit, do we have to do anything to keep it there?
How did DS1 get into space?
How does propulsion work?
What is an orbit?
What causes an orbit to happen?
What is gravity?
What is a satellite?
How do spacecraft use an orbit to move from planet to planet?
What is thrust?
How does speed affect an orbit?
What could cause an orbit to fail?
What is retroburning? What is aerobraking?