If you're unfamiliar with working in a linux environment then we'd suggest you start by trying to learn some basic shell commands(see links below). The command line interface is referred to as the shell. There a various types of shell which use similar commands and syntax, and there are often more than one type available on a single system but we'd recommend getting to grips with the bash shell. Most advanced users have worked out how to make things happen with a combination of self study, learning on the job and advice from colleagues with linux experience. In addition to learning how to use unix shell commands its a good idea to try to learn about bash scripting or how to use another scripting language like python or perl. Scripting languages are essentially the tools of the trade when it comes to using a cluster like Rosalind. Not everyone will have to become an expert in these but there are massive gains to be had in becoming proficient in some form of scripting language. Chances are, you'll find that the time invested in learning these will pay off handsomely.
Bash Guide For Beginners http://www.tldp.org/LDP/Bash-Beginners-Guide/html/
Advanced Bash Scripting http://www.tldp.org/LDP/abs/html/
A useful bash reference. http://tldp.org/HOWTO/Bash-Prog-Intro-HOWTO.html
If you're a windows user the a good way to explore linux is to install linux as a virtual machine using something like virtualbox https://www.virtualbox.org/.