Next to the newspaper you can find the button for diplomacy. Here you can check and sort through countries by various tabs. You can see who is active, their relations, the amount of VP points they have, and the amount of provinces. Through here you can go to the Messages and Trades section.
Here you can communicate with other players, whether they are allies or enemies. You can also trade relations, resources, provinces or armies.
Diplomatic Relations is a major aspect of the game. It is your choice whether you want to declare war on someone, or become allies. The choices you make decide whether or not you will make it to the end of a game. There are many options to choose, so choose carefully.
Here are the 6 Relations you can have with another country.
War: You and the enemy country are at war. Any units that intersect will be in battle. Any units within bombardment range will be damaged, and any crossing into an enemy province and reaching the dot(City point) will result in you capturing that province. There is an automatic embargo on both sides.
Embargo: Cannot use the market to buy offers from the country you have been embargoed.
Peace: Markets are now open (no embargo). You do not gain morale penalties from being at war.
Right Of Way: Allows military access through an allied country without declaring war.
Shared Map: Allows military acccess through an allied country and the ability to see their units/land.
Shared Intelligence: A high command feature. Exactly like share map, but allows you to see the other maps shared with the country. Lets say for example, Morocco shares map with Turkey, and then Turkey shares map with you. You can now see both Morocco and Turkeys land/units.
You can give someone relations through the first page of Diplomacy by clicking the relations icon. It will give you choices of what to change it to. That doesnt always mean a player will return what you gave them. This is helpful when dealing with AI. If you send peace of Right of Way, they will usually return it within 2-3 days.
Another way is to set up a trade with a player in which you can offer relations in return for something else. This is much easier when dealing with active players. You cant trade with AI through this way.