There are multiple systems that you have to start up in order to have the ship operational,
Power up the 4 consoles(yellow button on each one),
-from propulsion console power the thrusters, boosters, airlocks and doors.
-from electronics console power up antennas and screens,
-from tactical console power up weapons when you need them,
-you can also power up reactors, turbines, solar panels and generators from energy console but you need to build these blocks first
There is a key bind that starts all the systems for debug purposes or if you can't be bothered with the procedures: press F1 to activate debug mode then F4 to power up all ship systems.