What has been done before which is closely related to your project?  Answering this question can be done throughout your project, and it will likely happen multiple times, but it needs to be done to help build your project and avoid others' pitfalls.  Your work will be built on the work of others, so cite accordingly.

At this point in a project you will also begin using other peoples' theories and models in framing your research.  This holds true whether you use an established theory, such as using evolutionary theory in an ecology project, or if you modify an existing theory, such as using theories describing the spread of diseases to help describe the spread of ideas through a connected population.  In either case you will want to justify what existing theoretical models you will be using.

Examples of theoretical models with regards to a topic such as atmospheric chemistry could be straightforward, such as the positive relationship between local air temperature and carbon dioxide concentration, or they could be be more indirect, such as the chemical pathways which describe how normally inert CFCs can break down ozone in the upper atmosphere.

Start documenting your references, and short paragraphs which help put them in context of the work you're trying to do.  Otherwise this goes from building up the body of scientific knowledge to being plagarism.