If you don't know what you want to say in your paper, you cannot start with an introduction nor conclusion.
What are your points? What is your thinking? What do you want the reader to realize?
I am fairly certain a writing teacher will give you details, but from my thinking, you ought to have a pretty firm idea on what you want to say in the paper first. Putting it on paper then is the easy part.
Ah, yes, BobPursley is correct. Here's how I teach this concept:
DO NOT start any paper's first draft by writing the introduction! How can you introduce a paper that you haven't written yet?
Follow the writing process, whether you're writing for science, history, English, or any other subject:
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.)
Do you have a thesis yet? Have you done your brainstorming (and maybe research) yet? Have you written an outline?
Check in the Essay & Research Paper Level.
Here are a couple of links for intros and concls -- but really, they aren't much good unless you at least have an outline for the entire paper.
Effective introduction: Draw the audience in. Get them interested in your topic. Perhaps ask a thought provoking question or give a hypothetical situation to help illustrate your point.
Effective conclusion: Wrap up your paper. Sum up your main points. Answer any unanswered questions.