In terms of making it, I can't offer anything solid in the way of the actual scripts, but I think I can offer a bit in the way of process.
First, before anything else (after getting the software ok) make a sketch by hand of the map you would use for it.
Then, on the computer, build the map in a software of some kind, but don't add colour. Make it black and white. Don't make units, curves or anything detailed, keep everything with straight edges and objects as circles or squares.
The thing you have to remember is that every single thing runs on scripts. Generating the map is a script, putting a unit to atk or defend is a script.
If this will be a top down TD:
When making the units, stick with circles still until the basic is done. a top down is much easier to do expecially if there is nothing variable, as in something can only do one set of things, and doesn't have the ability to choose between options. With a tower defence this isn't too hard. Attack units have to be generated, by telling the computer to place pixels on the screen that indicate something is at one point on the map. damage, indication and removal are other parts. Then the computer has to know to remove the pixels from that part of the screen and put a new item on the screen in a different place. According to us, it's the same thing, but the computer has to be told to do it, and it's essentially different. That just has to reccur, with small changes in parts to tell it to move forward, down or up or left as you want.
Like I said, I can't offer much, but hope this helps.
