To earn credits, you'll need to trade with NPC stations. An explanation is on this page.
To start trading items, you'll need to have them first. You start out with a small budget (can be claimed by contacting an Admin) where you can buy stuff with. This budget will mostly be spend on life support for your ship and fuel. On the Aruma, you'll find a Medium Cargo Container and two Drills. You can use those to mine ores. To detect ores, you need to leave your ship and take your hand drill. An ore detector can be bought later. When you got some items to sell, you can enter the station and walk to one of the terminals. In front of the terminal you'll find a button panel with the following buttons: 'Confirm', 'Left', 'Right', Abort. Confirm is the 'Select' button. Left is for going left and up, right is for right and down. Abort is a 'Back' button, and when in the main screen to retrieve your credits. Throw the items you want to sell in the collector in front of the screen. If you want to buy items, you'll need to throw your credits in it. When you got everything done, press abort a few times until you get your credits.