It's important to have an idea what your PhD will be about (discipline, field,etc...). You can do minor changes once you're in the programme but for the purpose of the PhD application, you need to show that you have an idea what you're going to do.
1. Make a list of departments you're considering and look at their faculty interests
2. Once you got a good list, see if you can introduce something novel to any of their current research work
3. Get in touch with a few of the faculties (they may not respond though). Otherwise if possible, pop in the uni during their office hours and talk to them.
4. NEVER ask the professor for a proposal or ideas lol- he/she'll think you're a troll. The point is you go there, you show interest in the field, you know the topic, and simply need to verify that they would be interested.
Hope this brief guide helps.