Nico Nico Nii!~
Regarding 1: https://www.pokemon.com/us/account/events/ is how you can find a league near you. However, only the cards legal in the current modified format are legal. The sets that contain cards legal in the current format are listed here: http://www.pokecommunity.com/showthread.php?t=289800
2. That depends on what you want to do. For playing, the packs that are typically the best value right now are Dragons Exalted, Dragon Vault, Boundaries Crossed, and Plasma Storm, simply because those are the newest sets and a lot of the better cards in those sets are cards not everyone has yet.
3. I recommend starting out with a Black & White Trainer Kit to teach yourself how to play again (works better with a friend, though). Next, I suggest getting a Mewtwo EX tin (since Mewtwo's so heavily used in competitive play and is its own best counter because of its X-Ball attack) and then looking up information on the following 3 deck archetypes: RayEels, Darkrai/Hydreigon, and Blastoise/Keldeo. After looking up information on those, decide which one of those interests you the best, or if none of those do, look up some other cards from tournament legal sets that look interesting to you. I can't really recommend any of the theme decks for tournament play since they generally don't contain the best cards for competitive play, and when they do it's only 1 copy of that card. Getting good Trainers from them is a bit of a different story, however, as a theme deck may contain 1 or 2 good Trainer cards.
If you have any questions please post them here!