You learn R by try and error. For me, the best is to have a challenging task. I like to just try the different code and use it with my or other freely available data. The best learning experience is to do it with real data because these have the real problems.
Learn and Understand R
As a base, I would suggest that you print your one of the up to date Refcards. They give me always a fast overview and help me to get started again. Big advantage if you print them you see the code always if you are looking for something. Here you find some stuff Refcards and Cheatsheets by RStudio. In the best case, you also try to start with a good coding style to make your code easier to follow for others.
Joseph Rickert back in 2015 write a really neat article about learning R and collected a few links to free courses and Helping pages. Check it out here. Learning R: Index of Online R Courses, October 2015 by Joseph Rickert
I as a biologist like to learn the R with stuff I am familiar with thus I sometimes using pages like this R (and S-Plus) for Ecologists (Is a page with basic information)
If you want to see what is going on in the R Universe. Here is the Leaderboard, showing the number of packages and functions.
Always believe me R is fun if you know something and till then keep on going.