What are the requirements to join a doctoral program?

The requirements for doctoral programs vary from university to university. However, one thing that's universal is that you must have an undergraduate degree (basically, a bachelor's degree) before entering a post-graduate (master's or doctoral) program. Also, it's easier to get in than it is to stay in... you are usually required to pass certain tests fairly early on (usually towards the end of the first year) in order to stay in the doctoral program.