It depends a lot on the subject of your MA. For many Arts/Humanities subjects, you would need a proposal relating to what you wanted to study, and this would explain why what you're doing will produce original research. Ideally, you'd approach a university or more to discuss this with potential supervisors working in the right area, and they'd help you refine the proposal. For many Science subjects (and some Arts/Hums) you would find an existing project, and you'd apply to work on that, with a discrete area of research as your project.
The site jobs.ac.uk will let you search for PhD studentships, and looking at a few of those in your field might help you get some idea.
findaphd.co.uk (or .com? I can't remember!) is where you want to go! You need to decide on an area you want to spend the next 3/4 years of your life studying. Then if you can afford to self fund or not. If self funding, you can pretty much do anything but you'll need to organise a supervisor so speak to specialists in your area of interest to see if you can find someone willing to take you on and if your proposed research question is worthwhile. If looking for a funded position, you just need to keep looking until you find one in the area you're interested in. In my area (veterinary) most funded studentships are advised in January