You can activate whitelist in your servers dashboard. In the server settings, scroll down until you find the category "Game Settings", Once you find that category, you should see "Whitelist Disabled" and "Enforce Whitelist", Switch both of these to enabled and you should be good to go!
You can also type whitelist add [username] in the servers console to add yourself! Do note to not add the slash at the beginning in the console!
Mwuhh's post in How do you get the whitelist command? was marked as the answer