If the server is using DHCP, it should be assigned a static address or a static DHCP lease. It is preferable to assign your servers a static IP address.
Determine the ports required to access services on your server. For example, on a web server you would typically use HTTP - port 80. For email, you may use SMTP - port 25, POP or IMAP.
Create a port forward rule for all incoming traffic on the required ports to forward to the server as discussed in Port Forward Rules.