I need help! Fort Defense game

Hello,i need help to make the grid part of a defense game.
im going to make it so that there are pics at the bottom of the screen and when i click on it and drag it,a mesh comes from it and i can place it on to the subdivisons.What i need to know how to do is,how to set up the drag and drop system with the grid. also i need to get the 3d space of the cursor for the main gun,so that it can show up as a glowing plane on the actual terrain,so i can aim easier.

here is a basic Ex of how my battle system is going to look:

here is a Ex of the subdividson setup i want to make:

Well, for the 3d Cursor, look up Nilunder in google then go on the website “Nilunder.com”, click Tutorials then you should see “Mouse Position in 3d Space” in the lists of tutorials.
For everything else, I do not know XD. Hope that helped… And hope not its a waste of ye time.