Read the thesis requirements for your department. These guidelines will tell you what is expected of you, when choosing a topic and preparing your thesis.
Read recent theses in your field, both from your school and from others. This will help you understand what type of research is currently being done in your area of interest. This may help guide you towards choosing a topic, as you may find an angle of research that needs to be addressed from another recently published thesis.
Write down a list of possible topics, after researching the current master's theses in your field. You can take these to a meeting with your thesis advisor to talk about them in more depth.
Arrange a meeting to talk with your thesis advisor about current possibilities for research in your field. Your advisor will be able to tell you if any ideas you have sound reasonable.
Talk to other students in your department about your ideas and any cultural norms that are in place in your department. Cultural norms include the normal way that people go about choosing a research topic and developing a thesis. For example, cutting edge research may be encouraged ,or following in a professor's footsteps with research might be preferred. You will want to pay attention to these norms in order to finish your thesis successfully.
Organize a thesis group with other students from your program interested in working together on thesis ideas. Meetings can take place once every few weeks and you can work together towards refining any topic ideas that you come up, with while helping other students with their topics.