how do I erase mavericks and download yosemite

You don’t have to erase OS X Mavericks in order to install OS X Yosemite.

Once OS X Yosemite is downloaded and installed, existing copy of Mavericks will be removed automatically.

You should backup your computer before installing OS X Yosemite.

For info:

http://www.apple.com/osx/how-to-upgrade/