My App Store won't let me download anything I have an iPhone 4 and I have iOS 8

What are you experiencing? Are you trying to download over wifi or cellular? Many if not all cellular carriers have data caps on downloads (to avoid people blasting their data use with a single app). If you're trying to download over cellular, try to do it again when you're on wifi.

Beyond that, if you can give some examples of what you're experiencing - error messages, etc, it can help diagnose what's going on

If you haven't activated the device yet, you cannot do anything with it.

You have to activate iPhone to do that.