Return to the Available Modules dropdown list, and select "Engine". Size 1 only comes with 12 default engines at this time in Creative mode. Select from this list and place the same way you placed the Hull modules. Please note that Engines Must Face the Rear of the Ship!
Engine Subsystems must include at least one Engine (I, II or III), and it's helpful to add Engine Boost if you're worried about the cost (not really applicable in Creative Mode, but it is valid for Career/Standard mode).
Engines add Thrust to your ship, allowing it to maneuver in space. The amount of Forward Thrust must be greater than the ship's Mass in order to move. Below is an example of a ship that would be unable to maneuver effectively in space:
However, by adding a Level 2 Engine Boost (2 Volume), we're able to have enough Forward Thrust to maneuver our ship. Keep in mind however, that the Pitch, Side, and Backwards are all still very low, so I'll be adding one additional Engine in order to allow more agile handling.
Once both of my engines are in place, this is what my ship looks like:
The current stats of this ship are as follows:
Next, it's time to add our weapons.