Apple's Boot Camp and related Windows drivers will do this for you, somewhat automatically. Boot Camp is included with OS X and is located in /Applications/Utilities. OS X Mavericks (10.9.2) uses Boot Camp 5.1.2. The latter will non-destructively add a Windows boot partition to an existing installation of OS X. Should you choose to remove the Windows partition in the future, you must use Boot Camp for that task.
When you first launch Boot Camp, select the button, Open Boot Camp Help.
You need to purchase the Windows 7 install media. I could not find a digital download of Win 7 from Microsft but did find ones from ther sources.
For setting up BC see:
An external drive is not required. However, you should have one to backup your computer files. Bad things do happen
You only need a spare (~8GB) USB stick for those Boot Camp Windows drivers.
Microsoft no longer directly sells Windows 7. You will need to purchase it as a boxed license/DVD at a local/network reseller such as Amazon. You will need a DVD player. Although it is possible to download Windows 7 from Digital River as an .iso installer, it will only be valid for 30 days, and then you will have to purchase a license key ... if you still can. Installation of Windows 7 in Boot Camp is covered in the prior link I gave you, and familiarity with the Windows 7 install process in general.
You will need a dedicated external hard drive to back up your system using Time Machine. I would do this before any Boot Camp effort. I use a 1TB external drive to back up a 128GB SSD. Do not install the drive manufacturers software into OS X.
I believe you can add the Boot Camp partition into the Time Machine backup via TM system preferences. I you do that, you may wish to use a 2GB TM drive.