Bits of advice before you start a PhD:

1. Be clear as to why you are considering a PhD.
2. Think of what career paths you might wish to follow after the PhD.
3. Bear in mind that doing a PhD can be lonely, especially when you begin, and especially if you ar in a discipline that requires a lot of solitary study.
4. Doing a PhD is not just an academic enterprise. To get the best out of it (and yourself) you need to deal with the social side too.
5. When applying, try to deal directly with your prospective supervisor rather that the university’s administrative channels. Check out their work, try to speak with them on the phone. Try to speak with some of their students too.
6. If you get called for an interview, remember that although technically you are interviewing you, you are assessing whether or not the place and supervisor would be right for you as well.
7. Be aware that doing a PhD is quite unlike doing a taught course. A history of success in exam-passing won’t cut much ice if you cannot adapt to doctoral research.
8. Be prepared for the fact that once again, you are at the bottom of the ladder.
9. Although three years may seem like a long time at the start, in the context of a PhD it is rarely enough.
10. Ask yourself why you are considering a PhD (again, just to be on the safe side).

Source:

Your PhD Companion: The insider guide to mastering the practical realities of getting your PhD (Stephen Marshall & Nick Green)