Which version of Ubuntu should a newbie install (64 bit), 12.04 or 14.04?

Ubuntu 14.04 LTS would be good to start with. Here LTS means Long Time Support which is basically of 5 years. 14.04 doesn't have considerable changes as compared to 12.04 LTS , from a average users point of view, but it is definitely worth a shot.

I installed dual-booted ubuntu 14.04 along with my windows 8.1 3 days ago, and It's working very well. I followed this guide: http://www.everydaylinuxuser.com/2014/05/install-ubuntu-1404-alongside-windows.html, but didn't need to do the boot repair. Everything functioned after installing. Best of lucks to you :)

Neither. Try Mint, which has more drivers, a more windows-like menu, and is built on Ubuntu. Or Zorin. Try the live disk first, to make sure it runs with your hardware, before installing. If you have any issues, go to the forums for that distro and see if anybody else has that problem (or, you're doing it wrong).

I think you should go for Ubuntu 14.04 I am using this in one partition along with Arch Linux and Windows. The best thing about it is that there are few systems error which show upon new installation of Ubuntu 12.04 in some PC's but the new version has many bugs removed and changes too

First depends on your hardware, you should check if is supported by any of them. Newer versions could have more hardware support. If support is not a concern, and you want Ubuntu in particular. 12.04 and 14.04 are LTS (Long Term Support) releases, will have for years security updates and so on. But also, unless you install extra repositories, with a few exceptions like i.e. firefox, they will stuck with the versions of the programs they came with. So you will get newer versions of programs with 14.04, and in general, what was improved in the distribution too (for good or bad), like, i.e. easy mounting android drives. That alone should put 14.04 over 12.04 today. Of course, that don't mean that 14.04 is perfect. They took some debatable choices with the distribution ( https://tim.siosm.fr/blog/2014/04/25/why-not-ubuntu-14.04-lts/ ), and is not the only distribution in play (Mint, fedora, suse, mageia, elementary and a lot more are also newbie friendly, each with their own pros and cons). But could work for you, and work as a first experience till you feel ready to start exploring.

I agree with James Van Damme, Linux Mint is the best and most stable Linux OS that you will find. But if you want to stick to ubuntu download 14.04.2 version, burn it to a DVD and install it. It is a LTS and therefore the most stable.

I am a happy Ubuntu user and dual-booted with Windows 8.1. I just upgraded to 14.04 LTS and it's still working fine. Nvidia X Sever Settings support make me do it. Ubuntu seems like the easiest Linux distro for unfamiliar Linux users. So,I don't know what is the problem you are facing but I believe that it can be solved. It might help answerera  by providing details about the problems.