Where Do I Turn In My Coruscant Commendations?
The person you will want to turn in your Coruscant Commendations to is Merkus, whom is located under the Senate Plaza right next to the Cantina. If that isn't explanative enough for you, he is in the same subzone as all of the tradeskill NPCs, the auction house and most trainers. Below is a picture of Merkus.
By the time you reach level 15 or 16 you will have quite a few commendations piled up, especially if you were picking them as your quest rewards. Even if you were not you will still have enough commendations for at aleast a single orange weapon (which costs about 16). I highly recommend that before you leave this planet you pick up an orange weapon from the commendation vendor or at least some piece of equipment that will help you more than the commendations will just sitting in your inventory taking up space.