It's easy - easier than learning r (or any other stat package)!
In your lxde menu you are going to find the "synaptic" program, that is a package manager, which installs or deletes specific packages and their software dependencies.
You first update its software catalog by pushing its first button "reload" (should be like that, I have the Italian version so in English it might be different).
After a longish update, click the rightgmost button "search" (or "find"?) and look for r-base.
When found, click on the little square near its name and select "install package" or some similar command.
Then, resorting to the button bar, push "commit changes" (or "apply" the equivalent command), then probably OK again, then your package will be downloaded from the internet and installed.
You already know r starts from a terminal window, I'm sure you do.

Well, let me know how it goes. You could do everything from the command line, too, but maybe synaptic is easier and can show you the thousands of available packages!