Please refer to this article first for information about leads and how to create them.

1) If you are MANUALLY CREATING leads, you can assign them from the Add Person/Lead page.  Enter the Agent's name in the Assigned To field. It's an auto-search field, so start typing the name of the Agent, and then select it when it appears in the drop-down results:

You also need to make sure you grant the agent permission to access the lead page. To do that, go to the Set Access Permission section on this page. You can leave the permission setting to "Everyone," which means everyone with main system access can see this page. OR make sure you include the assigned agent in the "User+" setting. Click Add Another, find the agent's name, select it and save. 

2) IF you are IMPORTING LEADS, you will assign the leads directly on the import spreadsheet. Include the assigned user's username email address in the "Assigned To" column. Users will automatically be granted permission to view these leads once the file is uploaded. Please see this article on our Contact Import method for more information on how to import leads.