Obviously you need to select and record only relevant information, as you find it.
To be sure your information is reliable, you must also evaluate your sources, especially web sites. Do not assume everything on the 'Net is true and accurate!
Ask yourself:
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What information can I leave out?
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How useful is this source?
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How relevant is the information I have found?
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How credible is the information I have found?
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How will I record the information I need?
Electronic notetaking or 'copy'n'pasting is a useful skill here. But do not waste time copying huge chunks of information, or even whole web pages that you then have to re-read later to understand how it fits your assignment.
BEWARE: Plagiarism or copying information and pretending it is your work is both illegal and unethical!