For Prospective PhD Students
Are you sure you want to do a PhD? See why a PhD may or may not be a good choice for you:
Why Pursue a Ph.D.? Three Practical Reasons, by Philip Guo.
So, you are sure you want to go for it. See this classic talk for how to plan your PhD:
How to Have a Bad Career in Research/Academia Pre-PhD and Post-PhD, by David Patterson,
and then read this book for more career advice:
A PhD Is Not Enough!, by Peter Feibelman.
A major part of your PhD is presenting your work in scientific publications, and talking about it at conferences or job talks.
For the former, read this excellent article by Gopen and Swan, and, optionally, Strunk and White's classic.
For the latter, if you are not confident about speaking in public, take any chance you can to do it.
Is there a Toastmasters Club or similar organization in your area? Join it!