Step 1: Initiate contact with your trading partner.
Step 2: Select "We wish to offer you a deal" from the menu. This will bring you to the trading screen. On the bargaining table (the centre window) at the bottom are three buttons. "Clear Table", "New", and "Active". Click on the "Active" button to see what deals are currently running. Anything with a number in parentheses cannot be cancelled for that many turns. Anything else (including peace) can be clicked on. This will bring you back to the bargaining table. You can re-negotiate or clear the table to cancel a deal.
No, you cannot break a deal before 20 turns are up (unless you declare war).
Hope this helps.