Honestly, it doesn't matter. This question is on the same level as what programming language to start with. It doesn't matter, just pick one and start using it and learn.
I would suggest you use VM to start with. Install one in VMWare or VirtualBox and try to use it like it was your main OS. Find one you like and are comfortable enough with and install it on your actual machine.
I started with Ubuntu, moved to Mint, and then to Arch. Been with Arch ever since because I like it. My wife started with Mint and only uses Mint because she likes it. I have friends that run CentOS because they like it, others FreeBSD. It doesn't matter, pick one if you don't like it try another.
The point is that your learning. Since your on this forum, that is the main and only point. Others, like my sister-in-law, don't care. They want something simple, fast, and easy to use, so she uses Ubuntu and is happy. I like to code and muck about, so I use Arch and am happy.