You can pick any/all the leaves. I usually pick the biggest ones or if the plant is really bushy, I pick whole clusters of 4. You want to always leave at least half of the plant's leaves so it can stay strong. You don't eat the stems.
When the plant starts growing stalks of little white flowers at the top, prune the flower stalks off completely. This will extend the season of the plant and make it grow more leaves for you. Once it starts putting energy into flowering, it puts less energy into growing leaves, and if you let it go to flower it will die much sooner in the season.
Basil loves rich soil (like compost) full sun and lots of water. If you end up with tons, pesto is really easy to make, and delicious. Have fun!