Spacecraft and astronauts can move in the vacuum of space by using propulsion from rockets/jets of gas.

Newton's second law says that "for every action, there is an equal and opposite reaction".

Basically, that means if you throw something, no matter what, you'll start moving in the opposite direction.

Rockets achieve this by shooting lots of gas very quickly out their rear. By shooting that gas out the back, it's effectively "pushing" the rocket forward.

And, you are indeed correct that, once you're on a path, the only way to stop moving or change direction is to either use more fuel or to use gravity.

As for interstellar travel, even the nearest star would take on the order of decades to centuries to reach. But there are some really cool ideas out there for how we could try to get there faster.

One of them is a solar sail. It's essentially a really big, thin, and super reflective parachute that catches the momentum of light to accelerate.