Typeface is to design what salt is to cooking. It never feels that important but it can be the difference between an average design and a brilliant one.

Usually, I use this incredibly helpful chart for choosing typefaces: http://img.labnol.org/files/font-selection-chart.png

You could also take a look at these Smashing article for some Typography essentials:

http://smashingmagazine.com/2009/03/18/10-principles-for-rea...

http://smashingmagazine.com/2011/03/24/how-to-choose-a-typef...