Once R is installed, there is a comprehensive built-in help system. At the program's command prompt you can use any of the following:
help.start() # general help
help(foo) # help about function foo
?foo # same thing
apropos("foo") # list all functions containing string foo
example(foo) # show an example of function foo
# search for foo in help manuals and archived mailing lists
# get vignettes on using installed packages
vignette() # show available vingettes
vignette("foo") # show specific vignette
R comes with a number of sample datasets that you can experiment with. Type data( ) to see the available datasets. The results will depend on which packages you have loaded. Type help(datasetname) for details on a sample dataset.
This free interactive course covers the basics of R.