Writing the introductory paragraph can be a frustrating and slow process -- but it doesn't have to be. If you have planned out your paper well and thought about your thesis statement, then most of the introductory paragraph is already written. All you need now is a beginning and an end.
Beginning Sentences: Here's your chance to introduce your topic and grab your reader's attention. Starting your paper with "In this paper, I will" or "This paper is about" is common to the point of overuse. Try starting with a strong hook, that actually draws in your professor and makes him or her want to read more. In your research, did you come across an odd factoid or interesting quote? Or how about starting with an anecdotal story or humor? There are many ways to make your writing more interesting for both you and your professor, so don't be boring.
Middle Sentences: Usually, the middle sentences cover the points in your paper. Since you've already planned the order of your points, you already know how to place them in your introductory paragraph. You don't have to include every single point, but make sure the important ones get in there.
Ending Sentences: All the previous sentences have been building up to this: your thesis. Your thesis statement expresses the overall idea of your paper and show where you stand on the topic. This guide from San José State University has a great flowchart to help you write a good thesis statement, as well as links to other resources.
This is an example of an introductory paragraph, from a paper I wrote in library school (yes, that's really a thing). I start out with a broad statement that will hopefully draw in readers interested in this subject, then I narrow in on the specific topic that I intend to discuss in the paper. The questions outline the points I'm going to discuss about that topic, and then I finish off with my thesis.
Virtual communities--online message boards, forums, online games, social networks where users contribute content and/or communicate with other users--bridge time and distance and are a huge part of our current quasi-cyber existence. One interesting issue to examine in the behaviour of virtual community users is the phenomenon of lurking--ie. limiting online community behaviour to observation or only sporadic contribution--as it presents an interesting framework within which to discuss and evaluate the social implications of this particular technology. How do lurkers mediate virtual communities? What, if anything, prompts them to become more actively involved? As libraries move towards more kinds of virtual service, understanding the behaviour of less overtly active users and reaching out to them becomes increasingly important.