Having problems replacing battery in iPhone 4S

A friend sent me an iPhone 4S and said it worked. When I received it, however, it would not turn on. I thought the easiest and cheapest solution would be to replace to battery, so I ordered one. Once I opened the phone though, the battery replacement was not the simple choice I thought it would be . To top it off, my cat seemed to like playing with the tiny screws and I could not find them. Where can I get such small screws (and whatever other small parts that might have been lost), and what do I need to do to get the replacement battery in (hopefully that is still the problem)?

Thanks for any help.