Many books and on-line tutorials are available that provide a good introduction in R. Online sources:
CRAN website > Manuals. Official documentation. Provides an extensive overview, but might be sometimes technical. However, many examples are given.
CRAN website > Contributed documentation. Tutorials in different languages, in different sizes, and aimed for different audiences.
Cookbook for R by Winston Chang
This page is not intended as an introduction in R, but will provide a summary of some useful commands to start with data analysis in R.