AI is one of the tougher things to do I think. I would just start out with one behavior like bullet and play with that, the space shooter is a decent example for that. You will have to customize of course. There are other ways to do things like the other people said.
Here is a link for AI in the forums: http://www.scirra.com/forum/topic54735_post342167.html
Also, http://www.scirra.com/forum/topic45416.html is a very good place to find sample projects.