I have a new dedicated server and i am wondering what are the major must have security measures that i need to implement right off the bat.

I intend on learning everything but am looking for a jump start to protect my server from the most common attacks.

The box is a linux with ensim. Thanks in advance.

good place to start securityfocus.com

Make sure the only services running are the ones you need for hosting and the O/S. Also use Ipchains or Iptables to protect the box from the outside. Patching the OS is difficult because of the way Ensim ties itself to the OS and you are at Ensim's mercy there, note that they are pretty quick if it is a serious security concern or issue.

Besides these I would also suggest checking out the link posted above.

http://www.webhostingtalk.com/showthread.php?s=&threadid=33599