How can I keep something in space once in space?

Let's say you just launched something into space, with your amazing space launch system, wait, that's NASA's new rocket, SLS, if you didn't understand that joke then you clearly aren't educated about NASA. That was a joke too, by the way.

Now, your payload that you sent on your launch vehicle is in space, coasting upwards, but then it starts falling back down after it reaches its apogee, aka highest point away from earth. This is due to Earth's gravity pulling the object back towards it. To stay in space, you will need to achieve orbital velocity, which is the speed requited to get in orbit. For Earth, which is probably where you are, the orbital velocity is around 7,500 miles per hour, or around 3,355 meters per second. To achieve orbital velocity, it requires a lot of energy, this can be achieved with either solid or liquid fuel, either one works, and usually requires multiple stages on a rocket.

When you are not in orbit, but in space, you are either not orbiting anything or you are on a suborbital trajectory. A suborbital trajectory is like orbit, except while you are happily coasting in space around Earth, you will slowly get lower and lower and re enter the atmosphere, due to your velocity not being high enough.

Great! You are in a stable low earth orbit now, is that all? No, that is not all jimmy. You need to make sure that you are always in a stable low earth orbit, because while you are orbiting Earth, Earth's gravity will always be pulling on you, causing your orbit to lower over time. Because of this, if your orbit gets to low, you will be put on a suborbital trajectory, scheduled to violently re enter Earth's atmosphere making temperatures rise up to somewhere near 5,000 degrees F. Okay, now your payload is in a pretty much permanent stable orbit, as you created a reaction control system to raise the orbit whenever it detects that it gets lower. Your power will also run out on your payloads computer eventually, so you probably want a pack of decent batteries with some solar panels.

When you are in space, the temperature is very low, around -400 degrees F, so you want to use materials that can withstand that temperature, like aluminum, titanium, and more. When you want to de orbit your satellite/payload, you will need to fire your thrusters retrograde, meaning the opposite way which you are headed to.

Instead of doing all of this, you could have just bought a ride for your payload on another rocket owned by SpaceX, Rocket Lab, and more, but yeah still, congratulations.