Make a file writable in order to add new packages

I use Rstudio and try to add new packages through command console.

It fails to install it for this reason

install.packages("devtools"); devtools::install_github("hadley/rvest")
Warning in install.packages :
  'lib = "C:/Program Files/R/R-3.2.1/library"' is not writable
Warning in install.packages :
  cannot create dir 'C:\Users\myuser', reason 'Permission denied'
Error in install.packages : unable to create ‘C:/Users/myuser/Documents/R/win-library/3.2’
Error in loadNamespace(name) : there is no package called ‘devtools’

How can I make this file writable?