hi. You say you dont have a direct background for the research area of which you are interested in! Its ok!
You can look for people who are already in this area; and if there are studentships available--email them and ask, they may be able to provide you with links of where to find articles to read about it. I know friends who changed disciplines; they were also offered studentships, it is not impossible to enter another area. For me, one way to go about the proposal is to communicate with the prospective supervisor. It is easier to think on the proposal working with someone whos already done something in this area.
You can search for phd theses online (sometimes these are easily obtainable), even if the topic is not the same, you will have some idea of what a thesis looks like.
After all that, you will defnitely get some idea how to narrow down the topic.
cheers, satchi