The Unknown Planet
After you've obtained the four parts of the Star Map, a seventh destination will appear on your Galaxy Map: the Unknown Planet. Head that way once you're ready to begin the endgame, but be forewarned: you won't be able to return to any of the planets you've previously visited once you've made your choice. You might want to liquidate your assets and stock up on supplies before you head out to meet your destiny. If you wind up with a huge number of credits, you might want to pick up thermal detonators, life support packs, and as many deadly mines as you can carry, presuming your PC has a decent demolitions score. These won't be very important right away, but towards the end of the game having proper explosives can make some tasks much easier.
Objectives: Repair your ship
After your rough landing, get out of the ship and begin looking around; you'll soon be attacked by a group of odd-looking aliens; you might recognize them if you entered the prison during the "Unfinished Business" side quest that you obtained on Korriban. After defeating them, you'll get a bit of exposition regarding the island from a group of Duros that are similarly stranded on the planet. Feel free to start exploring if you'd like to do so on your own; otherwise, the next paragraph will give you a bit of an overview of the planet, and what your options are now.
In order to escape from the planet and access the Star Forge, you'll need to align yourself with one of the two surviving tribes of the Rakatan, the species which just attacked you. There's the warrior tribe, which has situated its enclave on the north beach, and the elders, which are descended from the priest caste of the Rakatan, and who have their enclave on the south beach. You'll need to travel through the exterior of an ancient Temple to reach the south beach. Without engaging in harsh judgments that might hurt the feelings of the involved parties, siding with the north beach warrior tribe leans towards the Dark side, while the priestly caste is slightly better for Light side characters.