There is a rumour that you can avoid infringing copyright if you change 10% of a work. This rumour is completely wrong. In most cases, if you change 90% of a work and copy only 10% you will still be infringing.
You infringe copyright if you reproduce without permission a substantial part of the original work. The words “substantial part” refer to quality, not quantity. If you copy a key part of someone else’s work you will be infringing, even if it is only a small quantity.
Just two bars of a musical work, four lines from a poem, or, in American rapper cases, one second of a sound recording, may be an infringing “substantial part”.
However, copyright does not protect titles, names, taglines or slogans. These are not sufficient to qualify as literary works.
Copyright also does not protect ideas themselves. It only protects the written form of expression of those ideas. If someone copies your idea but uses a completely different structure of words to express the idea, they will not be infringing your copyright.
Copyright also does not protect facts. A list of ingredients in a food product or a list of the people who live in your street are facts, not original forms of expression, and it is not an infringement of copyright for someone to copy those facts.
It is not an infringement to quote from a copyright work if the quotation is a fair dealing for the purpose of criticism or review, or a fair dealing for the purpose of research or study or reporting news.