1. Practice on PO a lot. Practice makes perfect. Then ask RNG'ers to make you the pokemon you choose to have in your team (for free most of the time).
2. I started playing VGC 2012 in Sept 2011 when the rules were first released up till VGC in March
3. No as I just started in September and I've qualified for worlds.
Sorry for the short answers, I'm kinda busy atm. Hope you'll understand.
EDIT: Showsni below has summed it up and expanded on my answers better.
Practice is probably the most important thing to do. Use an online simulator like Pokémon Online and you can play many matches against good players, and get a feel for what teams and sets work. Read up on the common sets for the Pokémon you want to use on Smogon.com, and read the warstories in the forums to see how people play.
My first Pokémon game was Blue, but I only started playing at all competitively after Platinum had been out for some time. It doesn't take too long to reach an at least semi decent level of play, and the rest is mostly fine tuning. Of course, for something like the VGC it's pretty metagame dependant, and since it's the only real competition in the year it's very hard to judge what the metagame is going to be. There'a a lot of luck involved!
No, I'd say anyone can pick it up and do well, certainly by the time next year's VGC rolls around.