Easy, install the virtual machine software and run it.
Choose the distro you want and the full DVD version form the webstire, I recommend Linux Mint, download.
Open the download with the virtual machine software and have at it.
You have to specify how much drive space to give to the guest OS, how much RAM, and how many cores.
For Linux 1 core and 1.5 GB of RAM should be plenty, you can always increase it later by powering off the guest OS and making the changes and restarting the guest OS.
When you install, your OS X window will appear like a screen on a generic PC and you run the Linux installer.
You need to treat the guest OS like any other, update it etc.
Create a "snapshot" using the vm software so that way you can revert your changes as your learning about Linux (or Windows or any other OS you want to install)
Linux isn't easy, but Linux Mint is the easiest for newbies as it installs almost as easy as OS X.