Determine the subject for your thesis. If you've had a title assigned to you, this won't be a problem. If not, choose an area of the subject you're studying that fascinates you. You'll produce a more compelling paper, if you're passionate about its contents.
Carry out preliminary research into the subject you've chosen, before framing the terms of your argument. Read the work of other writers and academics who have studied the subject. Seek out data and statistics to give you an idea of what evidence exists to support an argument from one perspective or another.
Compose a thesis statement that a reasonable person might disagree with, or at least find intriguing, before reading your work.
Set out your thesis statement to take a bold stand. State a fact from your initial research, and go on to say what your interpretation of the available evidence has led you to conclude. Avoid statements like: "There are both positive and negative aspects to decriminalizing the possession of illicit drugs." Instead, assert your claim -- such as: "After the publication of data that suggests illegal drug use in Portugal declined after decriminalization, the legalization of all drugs should be rolled out internationally."
Solicit feedback from your fellow students. Ask people if they think you've formulated a compelling thesis statement that would encourage them to read further. Most important, ask for the opinion of your instructor.