My recommended alternative is to borrow a Mac from a friend or family member.
I'm not positive if that setting is required in bios, but it really depends on how old your computer is.
Have you tried googling "virtual box install yosemite" ? This is outside my expertise, so I can't help beyond pointing you to look for the answer on Google.
Getting a Mac is really a priority for making apps these days. Many of my students complain how slow a VM or MacInCloud are with Xcode.