Start by Researching Your Topic
You can't start writing until you have an idea of what you are writing about! Find several articles that pass the CRAAP test.
Once you have located an article, look at the reference section to locate other studies cited in the article. This is a great way to find MORE articles. As you take notes, be sure to write down where you found the information. It is helpful to keep a working bibliography. Simply noting the author's last name and a page number in your notes allows you to keep track of where you got information from.
Create a Detailed Outline of your Entire Paper
A typical outline for a psychology paper includes an introduction, a discussion of the research (papers) on the topic and an evaluation of the research.
Once you are ready to write your introduction:
Introduce the Topic
Your first task is to provide a brief description of the research question. What is the experiment or study attempting to demonstrate? What phenomena are you studying? Provide a brief history of your topic and explain how it relates to the current research.
As you are introducing your topic, consider what makes it important? Why should it matter to your reader? The goal of your introduction is not only to let your reader know what your paper is about, but also to justify why it is important for them to learn more about.
If your paper tackles a controversial subject and is focused on resolving the issue, it is important to summarize both sides of the controversy in a fair and impartial way. Consider how your own paper fits in with the relevant research on the topic.
Summarize Previous Research
The second task of your introduction is to provide a well-rounded summary of previous research that is relevant to your topic. So, before you begin to write this summary, it is important to thoroughly research your topic.
Focus on hitting the main points and try to include the most relevant studies. You might describe the findings of previous research and then explain how the current study differs or expands upon earlier research.
Provide Your Hypothesis
Once you have summarized the previous research, explain areas where the research is lacking or potentially flawed. What is missing from previous studies on your topic? What research questions have yet to be answered? Your own hypothesis should lead from these questions. At the end of your introduction, offer your hypothesis and describe what you expected to find in your experiment or study.
adapted from: http://psychology.about.com/od/apastyle/ht/introduction.htm