Re-pot it in a larger pot. If it's in a three or four inch pot, put it into an eight-inch pot or larger. Handle it gently. Rough repotting often causes great harm to the critical tiny rootlets. It will get badly root bound in the small pot, if it's not already. Use potting soil or garden soil. Basil is fine in full sun down to filtered sunlight or bright shade. It will likely grow to something from one to two feet tall. Eventually, it will bolt, will form flowers, but when you see them emerge, if you pinch them off, it will last longer. Like many herbs, it doesn't need much babying, just adequate water, but err on the side of not overwatering. Most herbs taste better when abused. But pots dry out quickly, so a little water each day is appropriate. When mind reach the point where they insist on bolting, I harvest and freeze some roughly chopped in small freezer containers and make some into pesto and freeze. Small basil repotted into large containers grow and last for several months for me.
"Kitchen duty is awarded only to those of manifest excellence..." - The Master, Dogen