A good summary reflects a good understanding of a story. Depending on the length, you may need to read it more than once or take notes while reading if it's a longer piece of literature. Remember, a summary should be written in your own words and contain the plot, or major events of the story. Many teachers will tell you how short or long it needs to be. There are, however, summaries of the conflicts or themes, but usually these elements are written in an essay form.
I always tell my students to make a list by putting the first main event at the top of the list and placing the last main event at the bottom. Then ask yourself, "What happens next?", and write it down until you get to the last event you've already written down. Look back over the events you've listed and make sure they are the most important events that reflect the main issues addressed in the story. The last step of this method is to put your list in paragraph form in your own words.
Another method that works for some students is to write from a journalist's perspective. Use the five w's as a guide in writing your summary, the who, what, when, where, and why of the story. This method doesn't always work with all literature, however, so know the literature really well.
Good luck on your summary!
To write a good summary in general, you'd read the work, make notes on all major events, and write those notes up in an expository fashion. To help with this, notice where formal breaks are, and use those to help you create sub-divisions in your summary. For example, you could ask, "What happens in Act I? What happens in Act II?"
For plays, each scene is usually intended to communicate a specific point, so you could examine each scene—each time someone enters or leaves the stage—and ask yourself, "What happened here? What's the main event here?"
For a specific work that's well known, you can jump start your summaries by reading those written by others and comparing them to the works themselves.