Give citeRpacks() a list of packages you want to cite, tell it what to name your file, and if you're using RStudio, indicate whether to include an entry for the program (defaults to FALSE):
my_packs <- c("ggplot2", "Hmisc", "base")
citeRpacks(my_packs, "mybibfile", RStudio = TRUE)
To cite R itself, include "base" in your package list.
The function will save a .bib file to your working directory, which you can upload to reference managers that accept them for import.
There's a StackOverflow thread cited in the code that suggests sink() may cause issues if you have another package loaded that also uses it. This is the easiest way to get a well-formed .bib file, but run at your own risk.