There's a lot you'll need, but assuming you only have one Exchange server and one site (and that the Exchange server is not a domain controller):
-E-mail between your internal users should work out of the box.
-For external inbound and outbound e-mail, you'll need:
1. The Internet Mail Connector installed on the Exchange server.
-the Windows 2000 SMTP service must be disabled on the Exchange server for the IMC to work.
2. A DNS server for your Exchange server to resolve URLs on outbound e-mails. Your ISP will likely provide this, but if you have an internal DNS server for an Active Directory domain, you must use that one.
3. Your ISP's DNS server must have a domain name and A record for your e-mail server. Here's a scenario that would probably work for you:
your registered domain name: mycompany.com
your e-mail server's IP address: 1.2.3.4
your e-mail server name: mailserver
You should have your ISP create DNS records like the following on their DNS server:
domain: mycompany.com
(A) mailserver 1.2.3.4
(MX) mailserver 10
You'll also want a Pointer (PTR) record for your e-mail server at your ISP, since many systems uuse Reverse Querys as a security measure when accepting messages.
This sets your e-mail server's FQDN at mailserver.mycompany.com, which maps to the IP address 1.2.3.4. The MX record shows anyone querying your domain name that the mail server for the mycompany.com domain is mailserver, so all mail requests for the domain mycompany.com are sent directly to mailserver (the number 10 is a priority number that really only matters if you have multiple e-mail servers and MX records).
Since you have a small domain, you can take shortcuts, but this is the standard way to handle DNS records for mail servers.
Another option would be to have your ISP take all incoming mail for your domain, and then forward it to your mail server. You would still have to provide much of this info, but your ISP can walk you through any config info they need, and it can help fill in any service gaps in case you have server outages.