Welcome to the community! I would first take some time to go through the tutorials and get familiar with the tools. If the tutorials or forum posts are too complicated currently then you may need to spend some time getting better with the tools and features of making games with Construct. Hitting an enemy is just like hitting an enemy in any other type of game, you need to have collision polygons (hit boxes) and detect collisions with them and then react accordingly. Getting familiar with making simple games will teach you all you need to know for more complicated games.