Before you start writing packages…
Set up your system as explained in the R packages book by Hadley Wickham and Jenny Bryan. Contact me (Slack message, maelle@ropensci.org) if the call to devtools::has_devel() gives you an error that puzzles you.
Create a GitHub account, install git.
Also connect R with your GitHub account. See Managing Git(Hub) Credentials (and Managing GitHub credentials from R, difficulty level linux if needed 😅). Contact me (Slack message, maelle@ropensci.org) if the call to usethis::git git_sitrep() gives you an error that puzzles you.
Learn how to write functions. Some useful links: