Since you mentioned that your phone is iPhone 4S, and having some difficulty, I would recommend you to safe keep your logic board, there are some websites, like ebay and aliexpress, which they do sell the casing and parts, e.g. just without logic board or screen. I had performed repairs like this before, e.g. cracked iphone case / dented and got it from the ebay for it, with fresh set of screws, etc, but that can add up to your repair costs, one cost effective is to get those for parts, misc to salvage your phone.
Also get good quality screwdrivers, e.g. like from ifixit, a pretty good investment.