Firstly, delete any incorrectly entered accounts. Go to Tools > Accounts > Mail. If you've only got one account set up, it will probably already be highlighted. Otherwise click on an account (which you want to delete) in order to to highlight it. Click 'Remove' and then 'Yes'. If you need to remove more than one account, repeat as necessary.
Before proceeding further, make sure that you know the following:
1. The username for your email account. (That's usually the bit before the @ sign, but a few accounts - including BT - require you to enter your full email address).
2. Your email password.
3. & 4. The POP3 and SMTP servers used by your email service. Details for most ISP-based accounts can be found here
http://www.idl.uk.com/email/isp_settings.htm
or here
http://www.kitz.co.uk/isp/dns.htm
Now (from the point which you'd reached before in Outlook Express), click New > Mail and enter those details into the appropriate fields.
If the 'bt.com' bit in your post means that you're trying to set up BT Yahoo! Mail to work with Outlook Express, you'll find what you need here:
http://www.productsan...opic.do?topicId=25881
Chris