Quite often, I don't write my introduction to a research paper until the rest of the paper is written.  That's because what is in the introduction is a preview of the rest of the paper. 

There are a few ways to start an introduction. One is called the "funnel" method, which involves beginning with some general statements about your topic.  For example, suppose your paper is about single-parent families.  You might begin your paper with something like this:

There are many forms of the family, some large, some small, some simple, and some complicated. 

That tells your reader that you will be talking about families and it helps to get the reader's attention and interest.  From that sentence you can move on to more specific statements, leading the reader to the main idea that is the subject of your paper. 

The other way to start a paper is with a little vignette, a word picture or a little story that lets the reader know what the paper is about and gets the reader's attention.  Here is another example:

In a restaurant, there is a gay couple with two children, an American man and woman with two Asian toddlers, a single mom with a baby, and a group of teenagers with a dad.  All of these are families. 

Now, once you have a vignette or you have narrowed down from general ideas to more specific ones, you are going to need a thesis statement.  A thesis statement tells the reader what your main idea is and how you are going to support that idea.  Often, they take the form of "This is true because of X, Y, and Z."  Let's suppose again that you are writing about single-parent families.  A thesis statement on that topic might look like this:

Single-parent families can be functional, but research shows that children from these families are poorer, unhealthier, and less well-educated than those from two-parent households. 

With that statement, I am telling the reader what my main idea is, and I am giving the reader a preview of the ways I will support that idea, with sections on income, health status, and education. 

Once you have done your research and written the body of the paper, it becomes much easier to write an introduction.