Focus on maintaining the Command Center. The higher its level, the more buildings you will be able to build. Get it to level 3 to start seeing more available buildings. Maintain your port to make sure your docks are in working order. The more docks you have, the more space you can use to build an army. Then, expand by building a Science Lab (for development), an Arena (for battle practice), the Workshop (to create boost items), the Refitting Yard (to increase ship strength and capacity) and others useful structures.