Taking all my photos and videos off the iphone onto an External HD

Im almost at my storage limit. Ive tried numerous different ways to save the photos and videos on to my exteranal HD but it hasnt worked. On my last attempt I used Image capture. Can you suggest, step by step, how this can be done.

Also, I want to buy a new Mac book in exchange for this 2014 model and wish to clear everything off that.

how can I do that?