Getting stuck on the introduction paragraph is very common situation in any type of writing. Rather than trying to come up with the perfect introduction paragraph right off the bat, cut yourself some slack and focus on your thesis statement instead.
The most important part of any essay is a well-crafted thesis statement. Your thesis is a one-sentence summary of the major points you will make in your essay, and it's the key to a well-organized essay. It appears at the end of your introduction paragraph and again at the beginning of your conclusion paragraph. Write down whatever main idea you want to convey in your paper along with three major reasons (or examples). For now, that thesis statement alone will be your introduction.
Next, focus your attention on the middle three paragraphs of your essay. These paragraphs are the guts of your essay — where most of your final grade will be determined — and they deserve the bulk of your time and attention. Each paragraph should tackle one of the three reasons or examples that you listed in your thesis statement.
After you finish those three paragraphs, the hardest part is over! Now you're finally ready to tackle the introduction. The idea is to lead off your essay with a few sentences that will build up to your thesis statement. To get your creative juices flowing, imagine that you're about to hand your paper to a friend who's not in your class — what would you say to him about the assignment? Think about why you chose to write about this particular topic (or try to imagine why your teacher assigned it in the first place). Imagine that your essay is a TV news feature and think about what the anchor would say to introduce you. That's the sort of thing that you want to put into an introduction paragraph.
If you're still feeling unsure about your introduction, try writing two or three versions and then go find that friend of yours and ask for a second opinion.