Do you run the server yourself or is it hosted somewhere?
If it's not hosted by you, you can start by having strong usernames and passwords.
If you host it on your own hardware, obviously the strong passwords still apply, but you need to check a lot more. Check to see what ports are open, if there are any that you don't use, fix them.
Make a list of all of the software you use (Apache, Wordpress, your OS, any remote admin software, etc) and then search for exploits, security patches and major updates. Make sure all of the passwords and admin defaults are changed. Set admin/remote access rights to only local users if you can.
That's just a very quick list to get you started. I'm sure others will have more.
Hosting your own site and keeping it secure is a tough job.