So, the reason it's important to cite sources when writing a paper or thesis is because it:
allows you to show that you understand what others have said on the topic;
allows you to distinguish between your own ideas and those of others;
makes it easier to identify and look up your source;
acknowledges and/or thanks your source
shows that you are building on prior knowledge
increases your credibility and integrity
avoids plagiarism
Basics of citing
You cannot simply cite a copyrighted source. The citation must form part of your argumentation and may not quote more than is necessary for this purpose.