To buy fortifications: go to "Trade" >> "products in stock: buy and sell". Scroll down to " Defense Fortifications" and click "Buy", then enter the amount of fortifications you want to buy. You will have to wait (a game month or two), depending on your planets market situation for fortifications.
If you want to build your own fortifications instead of buying them, click "Corporations" >> "setup a new corporation", click "Defensive", scroll down to "Defense Fortifications" and click "Build".
It would probably be best if you bought them instead.