If you read the requirements it requires X number of defense points modules of which one is admin centre. It also requires Z amount of shields and Y amount of turrets or missile turrets.
You place the central Y piece down and plug the defense modules at it's corners (you need to rotate them). Right click the 2 defense modules and edit the loadout. If the requirements do not specify it, just install small turrets/shields to cut costs, not big ones. Above each module, there is a bar which you can set how many turrets/shields this slot can take 2/2 some can do 6/6.
You will have to buy the blueprints for the turrets (any), missile turrets (any) and shields and the defense modules.
Welcome to the world of the construction business which is lucrative especially in sectors where Xenon are blasting stations as the factions pay 100% to 300% higher price than normal
And when you can afford buy your own construction ship with full pack of construction drones. Some times I fly it my self building stations and is a spectacle