R Markdown is a tool that allows you to keep in a same document your code, your outputs (summaries, plots, etc) and, most importantly, your thoughts and comments. You can produce nicely formatted output in html, pdf or even Microsoft Word. It is extremely easy to use and will force you to document well what you are doing. R Markdown is a file format for making dynamic documents with R. An R Markdown document is written in markdown (an easy-to-write plain text format) and contains chunks of embedded R code.
Prerequisites: None, but familiarity with Windows helpful.