1. Use premium minecraft account to join.
2. If you are using offline account, just go to section "Files" in server.pro, select file "server.properties", find the line "online-mode" and change it to false, then you can join it after run the server.
You can use a web hosting minecraft server like.
Another way: Create a server using VPS or in your computer:
1. Create a environment.
2. Download software (Ex: Paper, Purpur,...).
3. Run it then allow eula.
4. Download and place the plugin into the "plugins" folder.
5. Run it again.
6. Forward...