In IQSS/Rbuild: Does not matter.

Resources

General

This link contains a number of truly fantastic cheat sheets, documenting everything from RStudio itself to data visualization and machine learning.
To read more about using R, take a look at the following website, built by Chapman and Hall
As always, it's worth referencing stackoverflow if you're ever confused. <!--
A list of useful packages can be found here. -->

Package Development

There's a wonderful cheat sheet for package development linked here. This also describes a lot of key components of the testthat package.
If you are looking for a very simple example of a package, the devex package can be found here. If you're having trouble understanding the workflow for package development, it's worth looking through the devex package and making sure you understand all its components. Better yet, you can practice using roxygen2 and devtools by creating a very small/useless package (2-3 simple functions)
It can sometimes be instructive to look through the source code and documentation for devtools.

Additionally, as you build larger and more complex packages, you might need a deeper understanding of package structure. For a slightly more in-depth explanation of package development, you'll want to reference Hadley Wickham's R Packages. For a serious dive into package mechanics, you should consult the official R Extensions Manual, which is published by CRAN. However, at least for mid-sized packages, this guide probably has given you most of what you need to know.