Creating a preliminary thesis is the first step. For this, you must already have a research question. Examine it carefully and do a little brainstorming as to what the possible answer(s) might be: make some educated guesses, in other words, and write them down.
You want to end up with a statement that isn't necessarily conclusive but gets you thinking and started on building one that is: a final statement around which your research can be focused. Let's use an environmental question fueling the Arctic National Wildlife Refuge drilling debate (circa 2006) to build a thesis statement.
Research Question: What is the impact of China's rapidly expanding economy on global oil markets and how does it affect the argument against drilling for oil in the Arctic National Wildlife Refuge?
Formulate a preliminary answer; add some supporting facts or observations-in this case both-as well as a preliminary conclusion, as in the following:
Preliminary Thesis Statement: The growth of China's rapidly expanding economy is radically changing the playing field in a global oil-market dominated for decades by the United States and Japan. Currently the second largest consumer of petroleum products in the world, China may very well become the largest, possibly importing as much as two-thirds of its requirement by 2025. This may be due to the growing Chinese auto market. It may also be due to an inability to meet established nuclear power plant construction targets. Regardless, the stakes are high. The question of where enough energy to satisfy global needs is going to come from cannot be downplayed. Despite the protests of environmental activists across the United States, the increasing needs of China to sustain their growing economy by contracting with nations deemed hostile by the United States may very well change the tone of the debate about drilling in the Arctic National Wildlife Refuge. It may tilt away from environmentalist conservation positions toward those who favor exploration and production.
This preliminary thesis provides some direction for your inquiry. In one short paragraph, four things are accomplished: