The sales funnel is accessible from the Funnel tab in the menu on the left-hand side of your screen.  This is your sales pipeline, which is a series of buckets that represent different stages of your sales process.  Much like dispositions, these buckets are completely customizable by your admin and will vary depending on your sales process.  Typically they begin with some sort of Prospect bucket and end with Sold.  

Leads will display in a list format in their bucket.  Simply click on a lead and the Contact Screen will be pulled up, allowing you to call, take notes, and disposition exactly like how you would a contact in a campaign.  

What is the difference between a Contact and a Lead?

Contact:  A contact that is in the dialer and has yet to be placed into the sales funnel.  Callable from campaigns.    

Lead:  A contact that has been dispositioned into a bucket in the sales funnel.  Leads are taken out of the dialer and are callable from the funnel only.  

Here is an example of a very basic sales funnel, starting with leads in a prospect bucket and walking down to sold.  Yours may differ, having less or more buckets.