If you're a new rider and don't know what you need, get a triple. You'll have a gear for everything and no big gaps between ratios.

If you get a double (compact or regular) and decide later on that you want to ride big mountains, switching to a triple is very expensive. But buying a bike with a triple costs $0-100 more than the double (standard or compact) when buying new. And if you decide to switch it to a double of some sort it's much cheaper to go that direction since the triple shifters will work on a double.