Whether you're writing an undergraduate or postgraduate proposal, it's vital you check your course and institution requirements prior to submission, since the word count and format can vary between universities.
According to usual practice, you'll likely be assigned a supervisor from your subject area, who'll guide you throughout the dissertation writing process.
Perhaps you are better able to select a more general area of interest, in which case you can start by looking at relevant journals and publications until you find a more explicit direction.
Make sure that you make notes of all publications that you use in your research, as you will need to include these in your bibliography later on.
It may be that the easiest way to begin to hone in on a specific topic is to go back through all of the lecture slides, notes and assignments that you have completed so far.
Was there a topic that you were particularly interested in?
The more reading that you do, the more you should be able to refine your research questions.
If you aim to address an area that is too broad, you will risk generalisation and run out of space in your word count.
Like the dissertation itself, your proposal will require an introduction, a main section and a conclusion.
As a brief guide: This is where you will need to introduce your topic.