Relations with your competitors and fellow players are controlled through the diplomacy menu. Change to the info mode, and then just keep clicking the right mouse button until you reach the top menu layer. Clicking the parchment with the left mouse button opens the diplomacy menu. Three icons will appear for each of your fellow players: a roll of parchment with a coin (trade agreement), a roll of parchment with a cannon (peace treaty), and a thumb.
The thumb indicates the player's current attitude towards you. You can influence this attitude by paying him tribute.