Hey Drakoniak,
The best place to start is by looking at the tutorial section of the site and getting use to the way Construct 2 works. There is one on turrets however I think it is link to mouse movement.
Also the FAQ section has a lot of example files to look at.
FAQ Check out the files on Tower defense-like turrets, Another example of enemies patrol and there is an inventory system the works well as a collector for the money/upgrades, I can't remember which one it is unfortunately, it uses a sprite with supermeatboy.
You can also use the search option, you general get good results, sometime the results are for the first construct so watch out for those posts
Good luck