You can find further introductory material (tutorials, full courses) for free in the following links:

1. R by Data Carpentry

2. R for Beginners by Emmanuel Paradis, and a Spanish translation, R para Principiantes

3. R manuals from CRAN

4. R basics, from from Learn X in Y minutes

5. R basics and Statistics and R, from Harvard University on edX

6. About Markdown, from Learn X in Y minutes