What size NC or Skylight filter for my new Nikon d3100?

Ok, I'm completely ignorant when it comes to cameras. I'm JUST learning and the initial learning curve seems to be a bit steep. Easy question for anyone with a clue though… I just purchased a Nikon d3100 and it was recommended that I immediately get a skylight, uv, or NC filter for it to protect the… Something.So, I'm looking at the filters and they are in all different sizes. I think I've got it narrowed down to 58mm or 55mm - but was hoping that someone could provide some real guidance to a newb.

Filters are lens specific, not camera specific. Turn over th elens cap and it has the size right there.

The most common lens, the 18-55 takes 52mm filters.

Personally I'm not too keen on protective filtering but if you feel you must… This is good and affordable:
http://maxsaver.net/HOYA-PRO1-D-Digital-52-52mm-UV-0-Filter-DMC-THIN.aspx

Not all lens caps are going to have the size stamped inside of them, and… What if you have a lens with no cap? You need to know how the filter size is specified on the lens itself. Look at the end where the numbers are. Look for what appears to be a 0 with a line through it. Following that symbol will be a number. That number is the filter size required for that particular lens.