Q: can i install windows 8.1 64 bit on new macbook 12inch 2015

you have to go out and buy a software disc but it can be done if thats what you are asking

For more info see:

System requirements to install Windows on your Mac via Boot Camp - Apple Support

Once you start to bring up Windows, remember that Windows does not understand that it is a GUEST on your computer.

Do not use any Windows-side Utilities to modify partitions in any way (e.g., Do Not resize the Windows partition using Windows-side tools), or it will clobber the partition Map and effectively wipe out your Mac OS X system.

Always the every helpful Bing/Google:

Good overall review:

http://www.anandtech.com/show/9136/the-2015-macbook-review

https://www.bing.com/search?q=new+macbook+12inch+2015+Windows+8.1

Apple had stopped supporting Windows 7 in 2013 though

http://www.macrumors.com/2015/03/20/new-macbook-pro-air-no-windows-7-bootcamp/

This is a Core-M and slower MHz so it will be limited. Would expect though that Windows 10 even the current builds Tech Preview would be more suitable (and you can download the ISO from Microsoft for now for free remember). I use 10-TP on i7-4771 and it handles very nice.