Every scientific paper begins with an abstract. An abstract should answer the following questions: what is the main research question?, what did we find?, how did we find it?, and why is it significant?. The abstract needs to be a short and succinct summary of the paper (4-10 sentences). It allows readers to quickly evaluate whether or not they want to read the entire paper. Because it is a summary, it is often the last thing we write when preparing a manuscript.
The first 2-5 paragraphs of a scientific paper are usually introduction and background. This is a space to explain the necessary background knowledge to your reader. This is where you explain the underlying concepts, history, and anything else your reader needs to know.
Your introduction should have lots of citations (or references) because you are referring to work or knowledge that has come before. Imagine that you are writing this section for one of your peers, preparing them to understand your experimental results.