I have linux server behind NAT but i can farward the port to LAN computer.
How can do that so that at least i can ssh
The standard port for ssh is 22.
You have to tell your router to forward a port (22, tcp) to your linux machine's IP. This is usually in a section called 'NAT' or 'Servers' in your router's configuration.
Most OSs have a firewall installed as standard. So you may have to open port 22 on your server machine's software firewall as well. (try 'ipkungfu' for a fairly simple interface to set up a linux firewall)
On the device which is doing the NAT you need to setup port forwarding.
This will forward incoming traffic on the chosen port to a particular server behind the NAT device.
How you set this up will depend on what device it is.