The engineering window contains everything you need to build structures and repair your own ship. At the top of the window, the current surplus energy for the system is shown as a quick reference.
The build section allows you to browse blueprints by category and build the structure if appropriate, if you can build the structure the build button will be active. Clicking through gives you details about the structure including the build criteria. This build criteria section helps you understand why you might not be able to build the structure. To build a structure you must be in a construction role ship, have the structure components needed in your hold, meet any skill requirements for the structure as well as have enough energy in the system and a suitable body to build on or around. Any build criteria with a cross mark indicated you fail the given criteria. The components required to build the structure are also listed in the details, along with if they are present in your hold or not.
Structure components themselves can either be purchased or fabricated using a factory. The consumables required for the fabrication are listed in the structures components section of the main engineering window, allowing you to browse what you need to fabricate which parts for a structure.
Lastly, if your ship requires repair as it has damage to its hull or systems you can start repairs using the repair section. Repairing is an idle activity that will take time and will stop once all damage has been repaired. Shields cannot be repairs and will instead recharge slowly over time based on the available power your ship has.