All that follows is In My Opinion.
Ok, sad truth. At the moment you don’t know what you’re interested in. Lots of poeple don’t. I didn’t.
If it’s all equal to you. . . Scary Truth: This is the moment that you make up your mind, or at least pretend to.
Choose schools based on reputation/rank, the best school you can get into, geographic location, pubs on campus, famous scholars, weather, whatever. Look at the department website. Find the faculty member-- preferably full, at least tenured-- whose work seems to sort of interest you.
This is the subfield you’ve desperately wanted to work in since you were 8. You even have a possible project in mind. Oh, yes, you do. At least two sentences worth of one, anyway. And this particular faculty member works here, who you think would be great to mentor this project, which is why you chose this school, oh yes it indeed is, in addition to its excellent facilities blah blah.
Second consideration-- check to see if you can that that subfield is still marketable. Or something up and coming and avant garde.
FWIW, I applied to all the grad school that were at the top of my field of interest (Tabletop computing) in December and by September, I was thoroughly sick of ever thinking of working on that stuff again and ready for something completely different. In my first year, I changed topics maybe 3 or 4 different times before finally stabilising on something I think is promising (social software).
What was most important to me during that time was that I really clicked with my advisor. We both thought in the same way, we both have a deep respect for the other person’s ideas and we both are excited about what I’m doing. The only way I figured that out was to talk personally to the people who would be advising me at the prospective universities.