RStudio requires R to be installed in the system.
But before you start installing R, you need to add the CRAN repository to get the latest version of R. For that, you can add an entry like the one shown below based on your Ubuntu version.
I am using Ubuntu 14.04 LTS (Trusty Tahr), so my entry will be like,
deb https://<cran.mirror>/bin/linux/ubuntu trusty/
Now you have to add one more thing to the above entry. The CRAN mirror you wish to use.
You can choose your favorite CRAN mirrors from https://cran.r-project.org/mirrors.html
I am adding University of California, Berkeley, CA as my CRAN mirror.
Now, open the terminal (Ctrl+Alt+T) and enter your entry based on your ubuntu version and CRAN mirror.
deb https://cran.cnr.berkeley.edu/bin/linux/ubuntu trusty/
Update the packages,
sudo apt-get update
Now we can install R packages
sudo apt-get install r-base
Once the above installations are done, you are ready to install RStudio
sudo apt-get install gdebi-core
sudo gdebi rstudio-server-0.99.903-amd64.deb
Note: You can check your IP address using ifconfig. The default port number will be 8787.
The username and password will be same as that of your system.