I would suggest to try something more straight forward for your first game.
A Tower Defense Game is a really difficult task, here you can see how far I got in 2 weeks work:
ubivis.de/td
to your questions:
-How do i creat a backgroud (one or two for enmies)
-- paint?
-How do i creat a turret?
-- either with a turret plugin or using a lot of math
-How do i spawn enmies
-- with events
-money, time, and a quickmenu
-- get a job, lose your job, paint :)