I don't understand if the tail is damaging the enemies or even does anything. I don't understand how to collect the blue orbs to make turrets.
Yeah! The tail does damage and pushes the enemies back
As for the blue orbs, they are the Splashes you can see on the top right of your screen!
If you want more turrets, you can buy them at the shop in between rounds, there is an arrow pointing to where it is (it's located in the center of the park/map)