You must always carefully cite the sources you are using in your papers, essays and theses. That is why it is of the utmost importance that you note down precisely which sources you are using, right from the start of your research. Note down the exact title description and page number of the source you are using. You must also indicate when you are using a particular passage literally (citation) and when your are refrasing a passage in your own words (paraphrasing).
Every student is expected to be able to manage their literature independently; this is an academic skill.