Once you've gotten your new Minecraft server, unless you get a permissions plugin, you'll need to be opped to run certain commands. OP, or operator permissions, is something that allows you to run administrative commands on your server. To OP a player:
Select the server you would like to op a player on from the panel.
Enter `/op <playername>` where <playername> is your Minecraft username then press the enter button.
We recommend later getting a permissiosn plugin such as LuckPerms later as permissions can be properly configured to allow things such as ranks.
NOTE: Do not OP people you do not know or trust as they then have the ability to destroy your entire server.