Emailing professor for PhD supervision
You should do basic research on the topics the professors is researching himself. Afterwards, you may either see whether there is a project around where you could supply your workpower, or you have an idea on a concrete project where he could extend research. (Keep in mind, he might need to get a funding for that).

If you do have an own project with independent funding, it might be easier - however, it should fit in the reseach area of the professor.

If you just come around, most professors I know simply will skip your application, as the normally have sufficient requests and work to do.