Hello all!

I'm new to this forum and I got a little question about hacking a personal computer or your own server.

I recently tried to hacking my own server (not in my lan to make it harder) and I gained total control of it by uploading a shell. (through uploading a .jpeg and editing etc...)

But now, I wanted to know how to hack into it through a port. I got the IP and let's say, there is an open port found (like a random one, 13455 for example).
How could I use that to my advantage?

To gain control, I should be able to upload a file or run a file to open up the port and listen to my commands and execute them.

But I don't really understand how to get there... Use an exploit? But I don't know why the port is open,...

Any help to get me in the right direction?

Thanks in advance!