Many times, not always-- I have a couple of good friends who are freaking brilliant who went from BS straight to PhD programs-- a person usually gets a masters degree before pursuing the PhD. Your masters need not be in the exact same area as the PhD (or bachelors) but it does need to be closely related.
--You will need to be an author on at least one publication. Yes there are exceptions, etc. But if you are in the sciences and doing the research and not at least the last author on something, that could be a problem. You should also be attending conferences and doing other active things at this point-- poster presentations, running a session at a workshop, etc.
--you should have great recommendations. It is really hard to get into PhD programs. Some programs have an emphasis on research, others on teaching (graduate students get funding by either being research assistants or teaching assistants). You have to be pretty special and stand out when it comes to what the school wants. You will need to click with the person who will be your adviser
--GPA and test scores
--The most important thing is that you love your area of study. The PhD takes a long time to get-- at least five years in most cases, and it can take many more. It is intense. Research and study of your subject have to be your idea of a personal Disney world, something that you want to eat sleep and breath. In order to avoid burnout you will need good mentors and support as well, once you get accepted. It also doesn't hurt to think about why you want the PhD and what you might do with it.