That’s an excellent question, one to which I only recently learned the answer. If you want your basil plant to continue growing and producing leaves, you should harvest pieces of it before it fully blooms. The tell-tale sign is the little spiky mound of pale green at the top of a stem. Simply clip the basil anywhere along the stem. New shoots will continue to grow around the area where you snip. You can also harvest large single leaves at any time, but you definitely want to harvest those almost-flowering shoots. A good rule of thumb is to never harvest more than half of the leaves on the plant. Your basil should continue to develop shoots and will become a wider plant rather than a tall one. Does that make sense I’m hoping to post a basil-harvesting tutorial soon with pictures, but it sounds like your basil is definitely ready for a first harvest! Let me know if this was unclear :)

Thanks!