If you don't mind the fact that they're dated now, start with Fallout then play Fallout 2. Then I'd personally recommend skipping 3 to come back to at a time you're completely finished with New Vegas. It's an okay game but not brilliant, the free roaming of it is fine and some elements of the humour are present but in general Fallout 3 is just an extremely sub par game.
If you can't get into older games (it's cool, some people don't like 'em!) then I'd go with New Vegas first, 3 is okay but it's just nothing special. Oh and if you do try NV first, pay attention to what people say, it's written like a true RPG, they'll give you hints and talking to people pays off but you can wander to your death easily enough. Pay attention to what's going on.