Here is how to make a bootable USB drive of the Yosemite installer after you download the full installer app from the Mac App Store.
Format an 8 GB USB drive which should be called Untitled and formatted as Mac OS Extended (Journaled). The installer should be called Install OS X Yosemite.app and should be in your Applications folder.
Run this in terminal and wait about 20 minutes:
sudo /Applications/Install OS X Yosemite.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled --applicationpath /Applications/Install OS X Yosemite.app --nointeraction
After the Terminal process completes you can eject the USB drive.
You can boot up from it by selecting it from the startup manager you get when starting your computer and holding down the Option key.
An 8 GB USB 2.0 flash drive is the minimum recommendation. The preferred hardware would be 16 GB or larger USB 3.0 drives. If using one of those, the process will be much faster.
Download the pdf version with pictures here.