Probably one of the best courses out there to get you started with R. It’s being offered by DataCamp in association with Microsoft. I have been taking this course (in spite of already knowing R, to get a sense of how good it is) because the reviews that this course is getting are spectacular.
Excerpt from the blog PROGRAMMINGR
The Art of R Programming is a must read for all intermediate level R programmers. It covers nearly every method of performance enhancement available and provides a review of key fundamentals that may have been forgotten or missed.
One point of note, this text focuses almost solely on programming – the statistical examples are a means to an end, not an end themselves. For that reason, this book is recommended for those seeking to improve the efficiency of their programming rather than their statistical acumen.