Step 1: Lay Of The Land
List of Distros:
Ubuntu - Recommended for beginners!
Debian - Nice all around distro.(looks a bit like OSX)
Kali - Hacker distro for advanced users.
Mint - Like Debian(but with improvements)
And many, many more(full list here)
Step 2: Install
1. Shut down the computer that you wish to install Linux on.
2. Insert CD/USB
3. While the PC boots press either F8, F11, F12, F2, or Delete to enter your boot selection menu.
4. Choose the CD/USB drive in the menu(every computer is different so "shop around" until you find one that boots something other than windows)
5. Now is the time to either test out the distro or install it, chose the install dialog to install or choose the "Live" dialog to boot a live version of your distro of choice.
6. The installation at this point is different for every distro.