The university uses a system in which admissions isn't under the control of individual professors, as is true in the US, for example. You need to apply and be accepted before he can really do anything for you.
Your reply should be to thank him, and say that you are applying. You needn't say any more at this time, as he gets a lot of such mail and likely won't continue the conversation until you are accepted.
This reply is actually more than you will receive from many (most?) professors whose university uses such a system.
Other places in which professors actually hire PhD students directly it would be very different.
You would also be wise to spend some time, once you arrive at a new institution, to consider your options for advisors. Often you don't need to choose one immediately and can explore the reputations of various professors with other students. An in-person interview with a prof is the best way to establish the relationship initially.