Holding your breath underwater

Question
Hi, this probably sounds like a strange question. How do you keep the water out of your nostrils when swimming? Everytime I hold by breath underwater, water tends to seep through my nostrils unless I breathe out. When I breathe out, I can't really control how much air to expel, so i usually run out of breath in 10 seconds. One of my friends could do a somersault underwater without the water getting up his nose. Maybe this has something to do with the shape of my nose. This is the main thing holding me back from swimming. I hope you could help. Thanks!

Answer
Josh--
To be honest there really isn't a time when swimming that you should actually hold your breath.  You should be either inhaling(hopefully above the water) or exhaling.  
Yes, some people can plug their nose either with muscle control, but most people have to blow out their nose or they do get water up their nose.  It's not a reason to stop swimming though.  Swimming is great exercise and is also alot of fun.
Hope this helps,
George