Making a tower defense game is a very different experience depending on what game creation software you're using.
However, usually the general premise is the same. You've got enemies who walk on a preset path at a certain speed. You would use Pythagoras Theorem to figure out the distance between towers and the enemies.
You would also need a health system for each enemy and other stuff as well.
Unfortunately I cannot give you detailed information unless you tell me what game creation software you're using.
