Library is not writable

I have this issue during package installation in R version 3.0.2 (2013-09-25) on an Ubuntu machine: install.packages("randomForest") Installing package into ‘/usr/local/lib/R/site-library’ (as ‘lib’ is unspecified) Warning in install.packages : 'lib = "/usr/local/lib/R/site-library"' is not writable How to resolve it?