You'll see tomato transplants at local nurseries before long. I have two bougainvilleas that are 5 years old. I have cut them back once. In fact, many say they don't water until the plant is almost wilting. Many say their plants color best when somewhat pot-bound.
Some gardeners use hibiscus fertilizer, which has a high last number.
One gardener who has colorful bougainvilleas applies 1 tablespoon of hibiscus food and 1 tablespoon of Epsom salt each month. Always water before and after fertilizing. There are special, controlled-release bougainvillea fertilizers on the market also. Prune your plant in the early spring. Pinch the tips to help boost color.