Usually you need to take a stand, either for or against. The thesis statement will state what you plan to proove.
Sometimes, before you can write a thesis statement, you need to do your brainstorming and some research, too.
Follow the writing process:
Prewriting: Brainstorm, research, plan, outline, thesis statement
Writing: Write first draft by starting with section II of your outline; write introduction after the body of the paper is written; write the conclusion last.
Polishing: Revise, concentrating first on the body of the paper, then the intro, then the concl (revision = making sure ideas are logical and sequential and support your thesis); proofread (spelling, grammar, usage, etc.)
Check in the Essay & Research Paper Level.
Your thesis statement must include factual information plus your position/opinion/stance. Without your position on the topic, it isn't a true thesis statement. So think of this sentence as the angle you want to take on the topic and what you intend to prove by the end of your paper. (If your statement is simply factual, then there's nothing to prove!)
This is one of the very best places I've seen online to help students write good thesis statements. It shows you sentences that aren't thesis statements and how to turn each one into real thesis statements.