In all honesty, most of the modern distros are pretty similar. I'd recommend one of the major ones (Ubuntu, OpenSUSE, Fedora, Mint) which all feature regular updates, large software collections, huge user community, and great support in general. I use OpenSUSE myself, mostly because I am a KDE fan and I think OpenSUSE has the best KDE implementation.
This site: http://virtualboxes.org/images/ seems to have ready-to-use images for VirtualBox, so you won't even need to hassle with the install. Try them out, and stick with what you like. That should get you started
Let the flames begin...
I'd recommend a debian-based distribution other than ubuntu (for several philisophical reasons). I really prefer debian sid based rolling distro's (such as Siduction or AptoSid) using the "smxi" script (google it) for updates... Some may think that's a bit hardcore for a beginner - although it's a great way to learn, and virtualbox is very easy to install/maintain with smxi, as well as graphics drivers. If that is hardcore for you, I'd probably recommend Linux Mint Debian Edition - based on Debian testing... ( http://www.linuxmint.com/download_lmde.php ). If you use a release that isn't a rolling release, make sure you have a separate partition for /home, as that will vastly simplify upgrading and/or re-installing...
...and now... make sure you use VIM as a text editor and not EMACS!!!
I've used a bunch of them both for business and for hobby, and I would tend to rank Fedora at the top and Ubuntu at the bottom. Bear in mind I am a complete non-GUI user when I am using linux.
"Every time I pitched it was like throwing gasoline on a fire. Pkkw! Pkkw! Pkkw! Pkkw!"
- Ebby Calvin "Nuke" LaLoosh
I'll add that if I'm going to use Linux, I want one that's GUI-friendly and is known for supporting lots of popular software (whatever that may be). Bonus points if it has a 'slick' look/feel to it.
I'm going to start by giving Mint a try. Pretty nice; I have two 24" monitors, so right now I have Windows 8 on the left, and full screen on the right I have Mint running through VirtualBox. Pretty cool.
Hmm. Well, I was. After installation finished, it said I needed to restart before file changes and whatnot would be preserved. I clicked to restart, and I've just got a black screen in VB right now.
*edit* I rebooted the 'machine' and now it's loading up. Weird.