Every sprite has it's on collision box. Set the collision between two objects on the event sheet.
If you need any help on how to use the event sheet, there's a bunch of really good tutorials here on the site.
You should also take a deep breath and take a time to read the Construct2 Manual before rushing into trying to use the software.
here are a few useful links:
https://www.scirra.com/manual/1/construct-2
https://www.scirra.com/manual/75/how-events-work
https://www.scirra.com/tutorials/all