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)
Reverse SSH if you dont have access to the router
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.