Navigate to the Dashboard.

Select the Lead Setting tile under the "Lead" tile set.

Select the Notifications/Assignments tab in the left hand navigation menu.

You may setup lead notifications for the Automatic Lead Assignment for Internet and Finance, Management roles, and Web Form Notifications.

Toggle the specific lead type to On.

Select the person or people assigned to that lead from the drop down menu.                           

Click Save.

Now that the rules are assigned, when a new lead comes in, that lead will automatically be assigned, or will give you the option to assign that lead to the appropriate person. The person assigned to the lead will receive an email indicating that they need to take action. 

NOTE: You will need to have a Default Sender setup in Lead Settings. This is shown in the help for setting up and Auto Responder. (How do I setup an Automatic Lead Response using the default template?)

If you reassign a preexisting lead, the new salesperson will receive email notification that they are now assigned to that lead.