Create a bootable usb from Yosemite.app So my problem is that my mac is not booting up. Also,the OS X recovery HD is not available. Also, the internet recovery is not detecting my WiFi .. so i intend to clean install Yosemite on my mac. I have the Yosemite.app and a boot-able USB and windows. But i don't know how to install from the .app file . please help
Check if your Windows machine has got a DVD-ROM or not.  If the computer can run Windows, it possibly can run Ubuntu or other Linux Live CD as well.  You cannot do something in Windows does not mean you cannot do so in other operating systems.

Read this:

https://askubuntu.com/questions/550591/create-yosemite-usb-dvd-installer-on-ubuntu

It gives a link to make a bootable USB out of 10.9.  That shall be the same for 10.10.  The trick is: Once you have got the .dmg file, you can make the boot disk as if the USB key is a sizable DVD-ROM.

However, make sure if your USB key has got enough disk space.  The installation file is less than 6GB, but the expanded 10.10.3 files requires 10.34 GB and my 8GB USB key thus failed to install into this morning.

Wish you good luck.