D3 is totally correct, but you don't really have to be careful, just grab whatever basil you need off the top. Frequent harvesting promotes growth and it'll always grow two more branches from wherever.

I've got some basil seedlings starting now. It's a useful and easy plant to grow. Only thing I noticed is basil hates water logged soil. It likes water but it likes to drain quick. Like most plants basil is way easier to kill from overwatering than underwatering.