Well several factors can be involved in this. One could be a firewall and the other could be if your computer is behind a Router doing NAT. And the other question is do you want them to have SSH or X-windows. The latter is going to require a VNC application.
If your Linux box is hooked up to a router you will need to forward port 21(SSH) on your router to the internal IP address of your Linux box. Are you sure the SSH daemon is even running. Open up a shell prompt and type:
ps -A | grep sshd
I dont know which version of Linux you are using, I am assuming Red Hat from your previous posts. Not sure what Red Hat uses for their default firewall. Try typing setup at a shell prompt. You should get some configuration options for your Firewall.
If Red Hat uses Iptables by default try posting the results of:
iptables -L That will give you all the rules of all the chains.