It's necessary to contact other civilisation. To talk to them first you would have to find them. After you get close to a habitated planet - you would be able to start your negotiations.
There are different species in the Galaxy, each with different life philosophies. Some loves religion, other want just more money, another - explore planets. Your kind also has it's attitude - it depends on your deeds in previous stages. Depending on your attitude - your interlocutors would act differently.
When you get yourself an ally - new option would appear. You would be able to ask him to attack hostile empire or to lend you his spaceship. Sometimes he would even send you some money.
In return you would have to sometimes to fulfil his wishes - he would ask usually for help in defending something.
Acts of aggression
When you start war, attack other civilisation - you can soon expect retaliation. Your enemy will start to constantly attack your planets and will attack you when you're on his territory. And forget about recharging your health and energy bars in his cities.
Some of you enemies would even ask you for money or otherwise they will attack you. In my opinion it's better to don't pay and just defend your planets.