I guess the first thing is to actually want a PhD. Not everyone can finish it. I have read somewhere that the attrition rate is actually at 50%: this means that half of the people who start graduate school end up not finishing it. And anecdotally, that is true. Half of my friends who I first met were PhD students in the same program as I was ended up quitting the program.
So what does it mean to want it? Well, it pertains to a very specific type of want. Something that can at times, be actually characterized as masochism. If you're in a PhD program because you decided to apply to graduate school immediately after graduating from undergrad since you have nothing better to do, then that's not wanting it. If you're in a PhD program because you were curious what it would be like to live in a foreign country and is using graduate school as an excuse to be a foreign student elsewhere, then that's not wanting it. Believe me, I have personally known individuals with these reasons, and sure enough, they left the program afterwards.
I've told other people that in order to succeed in graduate school, one must have a big ego. Yeah, I do admit, I have one. The thing is, graduate school is not for everyone. This is an environment where people have big ideas, and there are other people out there to trash and destroy these ideas. Unless one has a big ego, one probably cannot handle the cut-throat environment of graduate school.
So yes, graduate school is not a walk in the park. There were times when I thought I myself cannot handle it, due to discouragement, frustration, and such. But hey, the pay-off still was worth it. I've had to work long hours at times, sometimes even during the weekends, sometimes running statistical analyses on Sundays, preparing lab meetings and such, answering tough questions from my adviser and committee members, and yet, at the end of the day, I think I still am glad that I did it. It was fun, from the nerdy point of view, but yeah.
So yes, the first step to succeeding in graduate school is wanting it. If one has other motives than that, then chances are one won't succeed and drop out instead.