It actually depends on how do you want to learn/use it. You can use it in a complete graphical spreadsheet environment. You can also use it as a programming language. Of course, you can mix both of them (and that is the best thing to do).
If you know the S language (the language for SPSS), you already know R. There are some small differences, but that shouldn't be a problem. For a spreadsheet like environment, you can use RKward (apt-get install rkward). For a complete and powerful IDE, you can use RStudio (http://rstudio.org/). The eclipse plugin StatET is also very popular. R is distributed in small, independent packages with base/core. In Ubuntu, the packages are named as r-cran-PACKAGE. e.g., lattice module is packaged as "r-cran-lattice". There is also a meta-package called r-recommended which installs most used modules for R. For reference, you should also install r-doc-html or r-doc-pdf. Those are installed in /usr/share/R/doc/manual/. The r-intro.html and r-lang.html are the most important manual in this folder. -- M. Nasimul Haque Appliansys, Coventry, UK http://www.nasim.me.uk -- Ubuntu Bangladesh https://lists.ubuntu.com/mailman/listinfo/ubuntu-bd
