Essay is the most important subject in CSS and many fails in it.
I am going to explain briefly the whole process.
Research]
Focus
There are those who say that before you can research or write, you must first choose a focus and stick to it. While that is good advice in some cases, there are times when your focus should change during the research process. For example, you might decide to write on a topic only to discover a more interesting, more relevant, or more easily researched topic while trying to find materials on the original topic.
That doesn't mean reading every book on the subject; rather, you should ascertain what sort of materials are available on the subject at all. When you do your basic search, keep in mind the following indications:
• If you find an endless supply of possible sources right away, your topic is too broad, and you'll have to dig down into the subject more deeply to find a more suitable focus.
• If you can't find any possible sources after a serious search, your topic may be too narrow or too new. While this could serve as an excellent topic for a thesis in that it provides opportunity for original study, it is probably going to be inappropriate for an early-university or high school essay.
• The Internet is a fabulous source of knowledge. It is also a fabulous source of utter nonsense. If all of your sources are Internet-based and you can't find any book sources, you have to seriously consider the validity of the subject matter for a university essay. Even cutting-edge technology has books and articles available that describe the basics of the technology.
• If all of the sources seem to be written by the same person or group of people, you must again seriously consider the validity of the topic. It might be too narrow, or it might be generated by 'crackpots,' or it might be a great topic that has not been written about often enough. Discuss the topic with your teacher/professor.
• If you find a good source, search again under the author's name in case they have another useful book that you didn't find in the first search.
Assuming you don't have any of the above problems in your preliminary research, you should now be ready to choose a focus for your essay. In your notes, come up with a brief focus statement to help guide yourself. This doesn't have to be grammatically perfect, and if you wish, it can be in the form of a question.

The point is to give yourself a guide by which to judge research as you find it. For example, here is a fake topic (don't fret about what widget watching is, I just made it up):
• Focus: The life of Joe Smith (1856-1902) and how he contributed to the field of widget watching
o Good sources:
Joe Smith: His Life and Times, by Sally Superwriter
Widget Watching in the Late 1800s, by Michelle Bogus

Article in Widgets Monthly: Joe Smith, Portrait of a Widget Watcher, by Jean Doorknocker
o Possible Sources:
Widget Watchers in History, by Frank Diddledum (may or may not contain anything about Joe Smith)
Joe Smith's Studies in Moose Physiology, by Noreen Numpkin (may or may not contain anything about widget watching, but since the essay is also on his life, there might be good information here)
o Bad Sources:
Great Widget Watchers of the 1950s, by Herbert Hogswatch
Prehistoric Widgets by Frank Diddledum
Jane Doe: A Widgetress' Life, by Sally Superwriter (unless she had direct influence on Joe Smith, her information is probably irrelevant)
Article in Widgets Monthly: How to Spot a Widget at 500m, by Alfredo Frinkenhuven
Notice how the value of the sources change when the focus changes:
• Focus: Widget watching, what it is, how it has changed, who has contributed to it
o Good sources:
Widget Watchers in History
Widget Watching in the Late 1800s

Great Widget Watchers of the 1950s
Prehistoric Widgets
o Possible Sources:
Joe Smith: His Life and Times (only as it applies to widget watching)
Article in Widgets Monthly: Joe Smith, Portrait of a widget watcher (only as it applies to Widget Watching)
Jane Doe: A Widgetress' Life (only as it applies to widget watching)
Article in Widgets Monthly: How to Spot a Widget at 500m (depending on the content)
o Bad Sources:
Joe Smith's Studies in Moose Physiology
Notice that because the topic broadened to cover all of widget watching, the number of good sources increased.

During your research, you may discover all kinds of interesting facts about related topics. In the example above, you might have learned that Jane Doe was desperately but secretly in love with Joe Smith. But unless that love directly affected the field of widget watching, the information is irrelevant to the second focus. It is only relevant to the first focus if it affected Joe Smith; so if he didn't know about it, it probably isn't relevant. You must stick to your focus in your writing, and avoid throwing in random factoids, regardless of how interesting they may seem. Otherwise, the essay becomes too long and disjointed. It can be frustrating to not use what seems to be a good bit of information, but unless you can work it into your focus well, you'll have to learn to set such things aside.
Of course, if a bit of interesting information fits the focus, by all means work it into the essay!