Instructions
Befriend another race by giving them gifts. Typical gifts for other civilizations are resources, artifacts or even ships that can you spare to give them. Each time you give them a gift, no matter how many times you've given it in the past, your relations with the race will dramatically improve.
Travel to a planet of the race you want to form an alliance with. For the race to accept the alliance, you must have a relationship level of at least "Friends."
Click on the portrait of the race that hovers over the planet. A communication menu will appear.
Click the "Diplomatic" tab. Some options may be grayed out, depending on your relationship level.
Click "Form an Alliance" to engage in an alliance with the race. If they agree to the alliance, you will receive a confirmation window and a ship from the race as a gift.