Walla is a word used by retarded Americans who don't know any foreign languages and barely know their own. The correct word they are looking for is "voila", which loosely translates as "here it is", "there you go", or similar meanings.
Of course, the word voila properly pronounced in French sounds a bit like "vwalla", but with the "v" in front pronounced softly, almost like an "f". Being stupid Americans though, they can't pronounce anything which doesn't sound 100% English (not that they can pronounce English either) so in their incredibly lazy way, they don't even try to pronounce it correctly.
Want to see a stupid retarded American? points at self
So I was on a cruise around the world and we were heading towards American territory again. This lady on the ship asked me who the stupidest people in the world are. So I pointed at America and said "Walla!"