R Markdown extends the Markdown language. It lets you include nicely-typeset math, hyperlinks, images, and some basic formatting. The R Markdown Quick Tour and Introduction to R Markdown show you how to write an R Markdown report in RStudio.
For creating homework/project reports, check out this guide to learn how to use its most essential features.
R Markdown Cheat Sheet is a two-slide reference guide. (Search online and you’ll find various versions – see which one you like.)
For other useful resources, check out Learn R Markdown.
You can post your questions on the class discussion forum (@ Piazza).
stackoverflow is a popular forum for posting and answering questions related to programming. Most of time you can find similar questions already asked by others. Questions tagged R can be found here and questions tagged RStudio can be found here.