Putting Exchange 2000 server on Internet

Dear Experts,

we have Ex 2000 server working properly at our LAN. The server is connected to the Internet just to relay outside message and collect user emails via a POP3 gateway software. We have a lot of mobile users who wants to access LAN messages when they are outside the city or other place. Now the question is what is the process we need to go thru with our ISP to put our Exchange 2000 server on the Internet and our mobile users can access it from anywhere in the world. I know that we need a public IP address from our ISP but i want to know what process we should follow and what is needed other than public IP address.
Please help me as soon as possible…. this is becoming a pain in the neck.

thanks a lot