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