How can I set up an internal server that can be accessed on Internet?

Type 1: Using DMZ
This opens up a single PC completely. All incoming packets will be forwarded onto the PC with the local IP address you set. It is a security risk, since the firewall is disabled.
Type 2: Using Virtual Server
You can selectively forward ports to map to your internal server. Different port can map to different internal IP address.
Type 3: Using UPnP
If ADSL router, operating system and application (ex. MSN Messenger) support UPnP feature together, port configuration is handled automatically. UPnP is supported by Windows ME, XP, or later.