A PCC is a Personal Charging Case, small enough to carry in your purse. It has a battery that you charge through a USB port, and will recharge a 510 battery 4-5 times "on the go." Makes up for the 510 not having great battery life. It will also hold a spare atomizer and 2 extra cartridges.
The button on the 510 is a little more than halfway down, closer to the LED end than the end you suck on. I've put a LOT of effort into finding a way to press the button while holding the 510 between two knuckles like a real cig, and I just can't make it work. Maybe you'd have better luck! At any rate, it's really no big deal to press the button with the pads of your fingers. Became second nature after 5 minutes or so.
Juice comes in strengths ranging from 0mg (0 mg of nicotine per ml of liquid) to 36mg, in all flavors. It also comes unflavored (in various strengths) so you can add your own flavoring. The right strength for you depends on what kind of cigs you smoke, and how many.
510 batteries last 2-5 hours, depending on how often you use them. Definitely have at least two batteries, so you can charge one while you're using the other. The PCC comes in handy for this when you're on the go.
You can find e-cigs in retail stores, but they're generally the cheaper models, and you're almost sure to overpay. Ordering online is really the only way to go...