I am a newbie for debian, I used fc3 before. when I ssh to my debian's server, it doesn't work, how to enable it?

[root@localhost ~]# ssh <ip>
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@ WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED! @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
Someone could be eavesdropping on you right now (man-in-the-middle attack)!
It is also possible that the RSA host key has just been changed.
The fingerprint for the RSA key sent by the remote host is
16:db:86:a3:3b:50:46:81:2c:4d:11:2c:83:3a:26:6g.
Please contact your system administrator.
Add correct host key in /root/.ssh/known_hosts to get rid of this message.
Offending key in /root/.ssh/known_hosts:1
RSA host key for <ip>has changed and you have requested strict checking.
Host key verification failed.

You shouldn't run ssh as root even if it is encrypted. After all a man-in-the-middle attack is possible like the error says. If you really want to, remove the corresponding line from /root/ssh/known_hosts , that is, the line beginning with the <ip> you snipped off. It will then ask you to accept the new key when you try to connect.

I want to know how to access ssh by other user?
I understood ssh by root will cause some security problems.

Umm, what do you mean by how to access?
I usually do "ssh username@ip" as user. Also I have disabled root login in sshd_config. If I want to, I can still su to root when already logged in.

EDIT: btw, I made a typo in the previous post. The file is "/root/.ssh/known_hosts" I forgot the dot before ssh

1) when I did "ssh user@ip, the result was the same:-

@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@ WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED! @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
Someone could be eavesdropping on you right now (man-in-the-middle attack)!
It is also possible that the RSA host key has just been changed.
The fingerprint for the RSA key sent by the remote host is
16:db:86:a3:3b:50:46:81:2c:4d:11:2c:83:3a:26:6g.
Please contact your system administrator.
Add correct host key in /root/.ssh/known_hosts to get rid of this message.
Offending key in /root/.ssh/known_hosts:1
RSA host key for <ip>has changed and you have requested strict checking.
Host key verification failed.

If you'd have actually read what was shown you, the warning told you what to do.
"Add correct host key in /root/.ssh/known_hosts to get rid of this message.
Offending key in /root/.ssh/known_hosts"

If you know for a fact that the system is ok, you can remove the offending entry from your known_hosts file.

I have a problem, when I try to login to my computer via ssh from another one I get the following: "ssh: connect to host ip port 22: Connection refused", I know that is because the service is not active, I want to know how do I enable it. Anyone can help me?

Yes, if you stop bumping two-year old topic and make your own.