Whenever you get in game drops and loot. Take them back to ESD or a frieghter in space and sell the items to them. That usually nets the most Energy credits. Otherwise, press I for inventory. Then on the replicator there is a sell button and you can sell the items but for a lower EC then a vendor. Just keep doing this and the numbers will add up. There is a exchange on ESD where you can buy better weapons and other devices for your ship and your ground toon/bridge officers. If you at level 10 and about to rank up, wait till you get your teir 2 ship and get MK 4 items.