Your introduction needs to accomplish a couple of tasks: it needs to tell the reader what you are writing about, it needs to give some background on the topic, and it needs to clarify your thesis. Don't forget to think of it as an introduction, as in being introduced to someone for the first time: you are introducing the reader to your paper ("Hello reader, I'd like you to meet my paper; it's about writing in psychology, and in it I summarize different viewpoints about the task of writing in psychology"; — notice how this presumes you know what your paper is all about: you should write an educated introduction, editing it to be even stronger after your paper is finished).
1. Do you need a "hook"? Not really. Raise the issue you are going to write about, but try not to sensationalize. A psychology paper is mostly scientific, so it is expected to be on the dry side. That said, you should convey why it is interesting, but this needn't be done in a quick, creative manner. Also remember that I, your teacher, am the one reading your paper; don't assume I'm dumb ("You probably never thought about __________") or easily awe-struck ("imagine that!"); I will read your paper despite its first line. Keep it clean, clear, and focused. And throughout the introduction it would be nice to see an expression of your interest in the topic, as the writer.
2. Should you refer to this world at the start? No, please spare that. Don't open your paper with phrases like "In today's world, …" or "More than ever before…" or "In today's technological age…" or "Modern society is…." These are clichéd overgeneralizations or extraordinary claims that are unnecessary. Your paper will be significant for its investigations, not for its imagined place in the history of the whole of humanity. Focus on your topic; take the reader there directly.
3. Can you use the old standard opening, "The purpose of this paper is…"? Yes, although formulaic, it will put you right on track and will present your thesis. If you don't know what the purpose of your paper is, neither will your reader; however, even if you do know, the reader also wants to know and doesn't want to wait and figure it out himself. So even if you don't start with this phrase, state your purpose somewhere in the introduction.
Here are some example lead sentences from real student papers that work well:
[For a film analysis] The characters in the film Finding Nemo (2003) depict many social-psychological concepts through their behaviors and beliefs.
[For a film analysis] City of God (2002) is a movie about growing up in the slums of Rio.
[For an experiment proposal] This experiment will test the validity of prejudice and institutionalized sexism.
[For an experiment proposal] To most people, first impressions are lasting impressions.
[For a traditional literature review research paper] Many different factors can influence the success of a romantic relationship.
(Note: I didn't say those were spectacular, but they're real and they work, at least as a first sentence. What they each do well is provoke curiosity.)
4. Should you include citations in your introduction? Yes. You are likely going to make some factual claims within your introduction, and those need to be substantiated with source citations just as you would in the body of your paper. Or maybe you are going to refer to a film or a book. If you are designing a data-collection study (an experiment or a correlational study, for example), your introduction will be everything you say prior to detailing the methods you use for your own study; and you are best off reviewing (in your introduction) some of the related published knowledge on the topic, which will help to set the stage for and justify your own study. There might be many citations in a section like this. Notice in the examples above, the films are cited (do this when you first mention the film to get it over with).
5. How long should it be? If you are writing a short college paper of about six pages, then your introduction will probably be a page at most. However, if you are writing a first-hand data-collection study, your introduction might be longer due to the discussion of background information. Most other papers should limit the introduction to the first one or two paragraphs (given that I generally assign 4-6 page papers).
6. Is it like an abstract? No, an abstract summarizes the entire paper; the introduction introduces the paper. So, for example, don't summarize your experimental methods in the introduction - you'll be detailing them in the very next section of your paper. Don't mention your findings if you actually collected data: save that for the results and discussion sections.
7. Are you making a distinct prediction, a hypothesis? If so, put it at the end of your introduction. Any paper that is proposing a data-collection study has, underlying it, a hypothesis (a testable prediction). Your introduction should be giving the background necessary to understand the topic and also the logic leading up to the specific prediction (that you are going to study to see if it is indeed true). Work toward your hypothesis and then explicitly state it at or very near the end of the introduction.
Some other sources:
I was going to defy convention here and not link to other sources. Most of what you'll find on the web are descriptions geared toward English-class essays, but not scientific writing. If you do search for other advice, make sure you search for science paper introductions or even specifically psychology paper introductions. Here are two:
Mickey Schafer's Writing in Psychology class site
Jeromy Anglim's psychology blog