You can't just show up really early, get an armband, and go about your day. Obviously you've never done something of this sort before, that would be just silly. Armbands are given out just a few hours (depends on the store) before the store opens, and given out in order of who's in line. So if you want to secure an iPhone, you'll need to wait in line until 6 PM, or until whenever they give out armbands, which for most Apple stores will be 1 hour before opening.
Also, once you get an armband, you usually aren't supposed to leave the line, even though your place is secured. Doing that is usually frowned upon, as armbands are usually just so nobody can sneak into line just before opening.