In order to use the Mac OS X Mail application in Leopard, all you need are these 4 key bits of information
• Your email address
• Your password
• The Incoming server address (for receiving mail)
• The Outgoing server address (for sending mail)
For the incoming and outgoing server address for your mail, you will need to get that from your ISP (Internet Service Provider)
**An example would be like a well known ISP called 1and1.com:
their incoming server address is: pop.1and1.com
and their outgoing server address is: smtp.1and1.com
If you can obtain this incoming and outgoing server information from your ISP (or if you already have it) along with your email address and password, you will be on your way to using Mac OS X Mail to retrieve your emails and sending out emails.
To help you further here is a how to knowledgebase from Apple which will guide you in setting up your first email in Mac OS X Mail in Leopard
If you run into any challenges setting your email account, this is the best how to guide from Apple directly.
If that is still not working for you, just let me know and I will respond to assist as soon as possible.