Just like your deeds in your past life determined this life, your next life is defined by your actions in this life, according to the law of karma. Karma is believing that what you put into the world will return to you. So if you do good deeds, you will be rewarded in your next life. Conversely, if you do evil deeds, you will be punished.
You can also choose to be reincarnated into a different life situation to learn specific lessons. For example, if you were a wealthy person in your previous life, you may choose to be reborn into a life of poverty to learn how to be humble.