Did you install the java package using:
install.packages("rJava")
If so and it still does not work you might have a version issue. If you use the 64 bit version of R you also need the 64 bit version of Java installed. See below post for more info:
If you managed to install Java you can run a browser with the below command:
rD <- rsDriver(browser = 'firefox')
remDr <- rD$client
remDr$navigate("https://www.google.com")
In the example a firefox browser is opened but you can also use "chrome", "phantomjs" or
"internet explorer".
For more examples see: