Win2K and Exchange 5.5

My company is using sendmail which being hosted at Verio. We want to migrate to Exchange 5.5 (currently using W2K active directory). I did installed Ex 5.5 on an advanced server W2K and able to make it work within the lan (create several users for testing purpose...sending and receiving emails). However, I need to make these users to communicate with the outside world such as the internet...etc. As of right now, I do install this email server inside the lan. How do I make it communicate with the outside world? What steps that I need to do for example protocol, connectors,...etc? Please help and be specific on your steps such as step 1, 2, 3...etc.
Thanks and looking forward to hear from you soon.