To send a single email out of the system, click on the Lead you wish to email from the Lead List. If that lead has an active email address listed, you will see a "Send Email" link below their email address.
Clicking on this link will open the email composition screen. Your name and email address will automatically be applied from your profile, but can be edited as needed.
Add your Subject and then compose the message. When you are finished, click "Send Email" at the bottom of the screen. Sent email will be recorded and logged to the History for the Lead it was sent to.