its easier than it seems. my dad grew up in a village (his father was a farmer) and taught him cause it was an easy way to communicate or get someone's attention. so he taught me cause i thought it was neat. anyway, sometimes it seems those who use two fingers in thier mouth can whistle a little louder than without using the fingers. my mom used to call us in the house whistling with two fingers. i swear the whole neighborhood heard it. anyway, they're both about the same loudness i think, and personally i think its much easier to whistle without the fingers. plus if your hands are dirty or you're wearing gloves, well, you get the idea.
okay, i'll try to explain how to do it as best as i can. eventually after practicing you'll just "find it." i promise. (it took me about a week of practice before i could start to get some decent whistling sound)
okay, you've gotta curl the front part of your tongue and place it just behind your front bottom teeth.
then (while making a slight underbite) place your two front teeth on the curl of your tongue and blow.
just keep blowing air while trying to keep that basic position with your tongue and teeth and eventually i promise you'll get it.
it helps when you actually watch someone do it. but it'll still take a bit of practice.
anyay, good luck. it definitely comes in handy when you want to get someones attention.