For any research project, one can identify "foundational" literature. Who first introduced the key concepts you are using? Who first drew attention to the set of problems and issues you are dealing with? Who contributed significantly to the development of your sub-field(s)?
You do not need to go all the way back to Plato. All intellectual history is related, all is built on top of previous knowledge. So, it does not make sense to invest a lot of time in tracing the intellectual roots of your research inspiration. However, almost any problem or issue is associated with just a few most important names.
For example, if you are writing about learning motivation, there is no need to cover a whole gamut of motivational theories from Freud to Skinner (unless your research directly looks at different approaches to learning motivation). However, you would not want to miss, for example, the pioneering work of Edward Deci (1971) or Mark Lepper (1973). How do you find those two? Use all of these ways:
Ask your adviser or other more experienced researchers. If they work in exactly the same sub-field as you, those names might be just at the top of their heads, or in their bibliography files.
Take a look at relevant textbooks and handbooks, and see who the authors cite in the appropriate section. Those are the names you need to look up.
Read any contemporary research on the subject; in most cases, the foundational names will be referred to at the beginning of each paper.
==== References ====
Deci,Edward L. (1971) “Effects of Externally Mediated Rewards on Intrinsic Motivation,” Journal of Personality and Social Psychology 18/1: 105-115.Lepper, Mark R., David Greene and Richard Nisbet,(1973) “Undermining Children’s Intrinsic Interest with Extrinsic Reward; A Test of ‘Overjustification’ Hypothesis,” Journal of Personality and Social Psychology 28: 129-37.