As someone who found himself in the same position as you, only a month (or two) ago...
If you're completely new to the wonderful world of VPS, I would recommend Ubuntu over CentOS; as I think you'll have an easier time finding answers to "newbie" questions. CentOS is a very quality operating system, but it tends to be found more in enterprise environments; thus, a lot of the chatter on the internet is b/w people that are experienced in Linux--which makes a lot of the info. challenging (but not impossible) to understand for a newbie.
Nginx vs. Apache is an ongoing debate that's constantly attracting more attention. Nginx seems to be the favorite web server for speed-hungry sites w/a lot of static files. But, I've found it more difficult to find config. & troubleshooting info. on the net that's prepared in a manner that a newbie can understand. Thus, if you're just delving into VPSs, I would recommend Apache for now, w/an eye toward migrating over to Nginx later (if the need arises). Apache is the most popular web server in the world, and you'll find plenty of documentation and answers to questions posed by other newbies.