Learn Bioconductor the hard way!
I recommend the DESeq2 tutorial at:

http://www.bioconductor.org/help/workflows/rnaseqGene/

I think this is a good start for you because:


DESeq2 is a very popular package. You'll learn how exactly a bioinformatician works.
The workflow starts from alignments, and show you how to connect command-line tools with the R-package
It shows you how to generate MA plot, PCA plot etc. You'll appreciate how easy to do such things in R.
Testing for differential gene expression is very important
You'll learn how R does the programming. For example, in R it's a good idea to create a data set object, and do something like this:


obj &lt;- .... # This is my R-object
obj &lt;- fun1(obj) &lt;- # Function 1 adds information to the object 
obj &lt;- fun2(obj) &lt;- # Function 2 adds more information to the object