Hi, i have an interview next week for a PhD and i presume i'll get asked whyi want to do a PhD. I know why i want to-because i find the topic interesting and to take the first step in my career, but are these reasons good enough and what else could i say to impress the interviewers?

Perhaps, you should tell them what kind of career you are interested in. I know that some universities offer PhD admission only to those who want to have an academic career - I mean becoming a professor. If that's yours, then tell that you enjoy teaching and why. It all depends on why you want to do a PhD and spend 3-4 years without good earnings.

The other reason is that a PhD gives you expertise in a particular area. It also builds your analytical skills.