1. Choose an Operating System. (If you have one chosen, you can skip this step.)
There are many , many operating systems for someone to choose from. From the original Windows all the way to Windows 8, The Macintosh OS's, OpenBSD and more Linux based systems than I can think of. Windows is known as being the most popular option for operating systems today with slightly over 90% the market share followed by the Mac OS's. The most popular option for a second operating system is some kind of Linux system, such as Ubuntu. Most Linux based operating systems are free to use and alot are open source. For more information on different Linux operating systems or "kernels" you can consult amdfangirl's "Consolidated Linux Distro And Software Guide" here. Steam has a section of games that can be run on Debian based Linux distro's, so using Linux for gaming is becoming more of a reality everyday.
2. Obtain a copy of the operating system you want to dual boot with.
To be able to dual boot a computer, you must have a second operating system in some form, whether it be a cd, flash drive, or even a hard drive with the operating system on it. Operating systems come as a special file type that is recognized by the BIOS on start up. These special files will most likely have the ".iso" extension on them. You can also have an operating system on a flash drive for transfer to the computer. Using a flash drive as a medium to transfer the operating system isn't for all computers though, as some motherboards can not boot from a USB port, please do a check before committing to a USB drive. If you have a hard drive with nothing but the operating system image or .iso on it, that will work too as long as you can boot from that hard drive. Note: If you will boot from an external source, having other files that didn't come with the operating system will complicate the process, it is best to dedicate those mediums to only the operating system until the operating system is transferred to a different source. Most operating systems will come on a bootable disc with the image or .iso on it. Linux distributions or "distros" will come on a bootable disc or usb drive the user sources themselves. Downloads of most Linux distros are available from there respective websites and most can be burned to a bootable disc using the windows default .iso disc burner. Some Linux companies will even send a pre-made disc if you pay for shipping and handling. Windows image Burner
3. Insert your operating system bearing device
Easiest step of all, just open up your default operating system, and insert your device with the operating system on it. If a windows autorun message appears, you can choose to run it and follow the given instructions from there, but I am going to explain how to do a boot install. Restart the computer. It is recommended you read further before restarting as step 4 requires a semi-quick finger and some timing.
4. Open the BIOS and select the boot location.
As your computer turns back on you will see a screen with the manufacturer of your computer or motherboard if you built yourself. This screen will come before the operating system screen. The screen will have some text in one of the upper corners, saying press (this) for boot menu. "This" is usally F8 or F12. I like to think rapidly tapping the key and looking like a maniac helps. If done correctly, you will reach a menu with multiple options on which device to boot from. Select the device in which you have the operating system you want to install. If you have a CD, it will be a disc drive. If you have a USB drive, it will be on of the USB ports. If you have it on a hard drive, it will be the respective drive. Common Boot locations
5. Follow instructions given from there.
If the computer boots correctly, you will see a screen of your operating system you are trying to install, though it might be a couple of seconds. Follow the instructions given to you. When it asks about partitions, decide how much hard drive space you want to allocate to each operating system . I recommend allocating more space to the system you will use more. If you are going to use one a lot more you can set one as a default boot option. When done with all instructons given to you, move on to step 6.
Here is an image of a computer ready to dual boot two versions of Windows
6. Celebrate, (if you want to)
You now have a computer that will dual boot two different operating systems. Congratulations!!