I recommend that you start with Linux Mint http://www.linuxmint.com/ or ZorinOS http://zorin-os.com/ .
You will notice several different versions to choose from for Linux Mint. Stay away from LInux Mint Debain Edition (LMDE) as that is for more advanced users. You will see Cinnamon http://cinnamon.linuxmint.com/ , KDE (K Desktop Environment) http://www.kde.org/ , MATE http://mate-desktop.org/ , and Xfce http://www.xfce.org/ . These are all different Desktop Environments and basically give a different visual appeal.
The Linux Mint Page will give you different choices http://www.linuxmint.com/download.php , if you are not sure what type of CPU you have, then you will want to go with the 32 bit version. Download the iso from the download page, then download LiLi http://www.linuxliveusb.com/ to make a Bootable LiveUSB (thumb drive) or use InfraRecorder http://sourceforge.net/projects/infrarecorder/?source=directory and bur it directly to a DVD.
place the USB in the USB receptacle (or place the DVD in the DVD ROM) and reboot your computer. Enter the bios with F2 or Del. and change the hardware boot sequence to CD/DVD ROM and/or external device before the Hard Drive. Exit and save.
Boot into the live medium, and click on the install icon.
If you want a distro that is a rolling release, then you may want to try Sabayon Linux https://www.sabayon.org/