I don't really think you have an emergency on your hands here. If the tree is in a pot, I assume it has soil in it, so just tend to it like any other plant for now. There is no need to repot it ASAP, and in fact, it may be a tad early for that. As for the whole bonsai thing, well, that takes a little more research, and pot size and type are dependent on what you have, and what you intend to do with it. Is your tree temperate or tropical, meaning is it an indoor or outdoor plant?

Joe