No one can give you advice without knowing what kind of tree you have and more about the conditions. Bonsai is not a kind of tree, it is the art of maintaining a tree (oak tree, ficus tree, elm tree, juniper tree, etc) in a miniaturized and artistic form in a pot.
A picture would help, so we could identify your tree for you and get a better idea of what is going on with it. And then you need to tell us how you are treating it - i.e. are you keeping it indoors or outdoors, how much light, how much/often are you watering it, etc. The better information you can give, the better we can help.