For Learning R Programming you have to download and install R Programming tool. It contains API and run time for developing your program.
R Programming fundamentals and then analytics is the main thing you have to learn. Its easy to learn R, but you have to do a lot of practice to become productive in R Programming.
You can also take training course in R to learn it quickly. Check R Programming Training Course
Other resources to learn R:
3: What are the Prerequisites to Learn R Programming?