Where you've registered?
As far as I know some universities offer their Unix based servers for online use of linux commands, but with very limited access. However, online practice is not a good idea.
On the other hand, simply download some VM (i.e. VMware or Virtual box) and your favourite linux distro. Install the VM on your PC and then install linux on the VM. In this way you will have full access to the OS and you will be able to run all user as well as admin based commands.