Without an introduction it is sometimes very difficult for your audience to figure out what you are trying to say. There needs to be a thread of an idea that they will follow through your paper or presentation. The introduction gives the reader the beginning of the piece of thread so they can follow it.
Many books recommend writing your introduction last, after you finish your project. This is to make sure that you introduce what you are actually going to say.
If your project changes in the creating process, it is important to make sure that your introduction accurately reflects what you will be saying. If, however, you have written a good outline and stick to it, then it is fine to start writing your introduction first. Just make sure in your proofreading that you have kept the thread consistent throughout the paper.
Start with a couple of sentences that introduce your topic to your reader. You do not have to give too much detailed information; save that for the body of your paper. Make these sentences as interesting as you can. Through them, you can hook a reader and get them very interested in the line of thinking you are going to develop in your project.
Then state your thesis, which may be done in one or more sentences. The length of your introduction depends on the length and complexity of your project, but generally it should not exceed one page unless it is a very long project or a book. The average length of an introduction is one half a page.