You can use Microsoft.WindowsMobile.SharedSource.Bluetooth but there is not enough documentation from Microsoft.
I would prefer 32feet.NET library which has the capability communication through bluetooth device.
Have a look at their official site,http://32feet.codeplex.com/
You will get samples using the library.
Moreover, you can also try with CodeForFun library,http://c4fdevkit.codeplex.com/
Hope this gives you an idea to start.