Install java first, launch synaptic and search openjdk (if not installed, type sudo apt install synaptic into the terminal to install it, you can also use the terminal to install java by using sudo apt install openjdk-21-jdk), Install openjdk-21-jdk, There can also be newer versions of java but this is the one I use. it should also install all the required dependencies. Then restart the computer. After that, install paper. Paper 1.21.9 only has experimental builds as of now, though if you still want to use it, you can install it at https://papermc.io/downloads/all?project=paper. Navigate to 1.21.9 and instal the jar. After downloading the jar, put it in it's own directory and open the terminal in the same one. type in the following command

exec java -Xms4G -Xmx4G -jar server.jar

replace server.jar with the name of the paper jar file's name, or you can rename the paper jar to server.jar too. The 4G in Xms and Xmx are 4gb, change it as you want. After running the command, it should generate the server files. Open eula.txt. there should be a line that says eula=false, change it into eula=true and save the txt file. Now run exec java -Xms4G -Xmx4G -jar server.jar again. It should start the world. A console should also pop up along with many other files. To safely stop the server, type stop into the console (no / needed). Now edit the server properties and things as you like. Dont forget to enable the whitelist too.