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 which player is active, their relations, the amount of their Victory points and provinces. Here you can also switch 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 or provinces. However, some options are only available when trading with members of your coalition. If you are not a member of a coalition or your trading partner is not a member of your coalition these options are not available.
Diplomatic Relations are 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 city will result in you capturing that province. There is an automatic trade 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. Additionally to shared map you also share espionage information, for example enemy units reveal by spies.
You can change relations to other countries through the first page of the Diplomacy panel by clicking the relations icon. This will set your diplomatic stance towards that country. That doesn't always mean the other player will return your chosen relation. If you change your relation to Right of Way towards AI controlled nations, the AI will usually return Right of Way after some days.
Another way of changing relations is to send a trade offer to another player. To do that, go to the Messages and Trades tab. Start a new conversation with another player and select new trade. You can now request a certain relation and give a certain relation in return. The receiving player has then the chance to accept or decline the offer. Accepting it will immediately trigger the proposed action. Other types of trades are possible as well, for example exchanging resources or provinces. But remember, some options are only available to members of your coalition. It is adviced to start a conversation before sending trade offers to other players. You can't set up trade offers with AI players.