Access sql server outside office

I have a static IP, beetel 440TXI ADSL2 Series router and Windows 2003 server machine. My internet line is connected to Router (DHCP enabled) which has one static IP. Now i want to assign an Statis IP for 2003 Server machine so that i can access my database/server from any where in the world? Please suggest me with clear details.

Thanks in advance.