Linux is different to OS X and Windows which both have a standard Graphical User Interface. With Linux you can pick your Desktop Environment (GUI). The most popular ones are Gnome, KDE and XFCE. It’s worth doing a bit of investigation first to choose which desktop environment is most suited to your needs/hardware then pick a Linux distribution. For example…
Kubuntu (KDE Ubuntu)
Xubuntu (XFCE Ubuntu)
Ubuntu (Standard Gnome Ubuntu)
Ubuntu based linux distributions are a good place to start learning as they are widely used, regularly updated, and have good documentation and support.