I will do my Master(public policy) in Bristol on October,2010. And, I have the idea of doing my PHD in OxCambridge after my Msc.But I want to know do I need to prepare a RP now for PHD application ? Is it difficult for a scholarship? I still have no idear how to prepare a RP.

probably, a PhD is not for you then.

Bit harsh there Smug. Ella, if you are looking to do a PhD as soon as you finish your Masters, which I'm assuming is ending in October 2011, then you need to start thinking about ideas for a research project. Once you have an idea, you need to make sure there is no current research going on that might solve your problem and ensure that it's a novel idea. Then you have to find a supervisor for your project, by asking in the department of the uni you wish to study in and checking other places too. Once you have a supervisor, then you can start to draw up a Research Proposal, ideally with help from your potential supervisor and you can apply for funding. Funding is very difficult to get and often you have to have your masters qualification before you can apply.
and a proposal should be the current info in your chosen field, what is missing, what your project will add to this, and why it should be you (and your supervisor) in particular who are the ones to solve the problem.

Good luck.