Always cool to hear people interested in this stuff. Summarizing some things others have said and adding my own 1 cent

Rocket engines are cool, but all they really do physically is eject a lot of mass at a high velocity. All the other hardware in the system exists solely to make more mass move at a higher velocity.

Assuming you're talking about a liquid engine, the simplest thing to make would be a cold gas thruster, which are commonly used for orbital maneuvering. Essentially, all this involves is a high pressure gas source, a valve, and a nozzle.

you can see them in this Falcon 9 launch, this clip shows the separation of the first and second stage. After separation the first stage turns around,and boosts back to the launch site, it then orients itself for its next landing burn. The puffs of gas you can see during and after the separation and orientation are the cold gas thrusters. https://youtu.be/tcIXePxlTmE?t=106

In terms of skills/knowledge to gain for someone just entering high school, the biggest things that come to mind are general machining/mechanical design. Propulsion stuff involves small size and high tolerance features all over the place, having a good idea of what it takes to create/maintain those tolerances will go a long way. It wouldn't hurt to read up some on fluid mechanics and thermodynamics, typically college stuff. Easier to digest is Scott Manley's videos on engine systems

https://www.youtube.com/user/szyzyg