“Breach the Cabal fleet base on Phobos.”
From where you land, just head forward along the ridge. There will be a few things to Scan as you proceed but largely focus on just going forward. For now, do not worry about any Cabal you come across. They are either dead or dying.
When you get close to the entrance into the base, you are likely to come across a Legionary that is still standing. He will go down very easily. Head inside the base that he was trying to protect or leave.