RE: Wanting to Play Again.
The rotation is occurring on September 3rd, and only sets from Boundaries Crossed-On will be legal for standard play (note that some tournaments will allow a new format called "Extended", which includes sets from Black and White-On, so always check with the tournament organizer/staff before you sign up). The best time to get into the game, I feel, is just after worlds, preparing for the new rotation, so you're in luck.
I would start by figuring out the kind of deck you would like to play before you start building one. There's a lot of archetypes in the format right now (and that's not strictly just types: many archetypes include a variety of different types of Pokemon; it's not "grass decks" that are popular, it's a specific archetype, Virizion/Genesect, which is) and a lot of different ways to play.
I would start by looking through the
Deck Garage forum and just picking out a couple of lists that seem interesting. Then, use an online simulator like PlayTCG (free, but manual interface), PTCGO (official simulator; you need to use codes from purchased packs to trade or acquire most specific cards, automatic interface), or TCGOne (free, automatic interface, but lacks some of the latest modified-legal cards). From there, you'll start to get a feel for both the metagame and how the decks you chose play. You can refine your choices to better appeal to the sort of play-style you find most enjoyable.
Once you've found something you really like, then you build it for real, and start play-testing it for tournament play.
And since you're starting from scratch, you may as well just pick up the staples while you figure out a deck. Here's a rough list of some of the most popular staple cards in the format:
- 4x Professor Sycamore (Or Juniper, but you can't play both and Sycamore will be in the format longer)
- 4x N
- 2-4x Skyla
- 2-4x Colress
- 1-2x Shauna
- 1x Pokemon Fan Club (for evolution decks only, for the most part)
- 1-2x Lysandre
- 4x Ultra Ball
- 2-4x Muscle Band
- 2-4x Silver Bangle
- 1-2x Surprise Megaphone
- 1-2x Professor's Letter
- 1x Computer Search and/or Dowsing Machine
- 2-4x Rare Candy (again, evolution decks only, but it's always in the format so it can't hurt to pick it up)
- 2-4x Hypnotoxic Laser (only with Virbank City Gym)
- 2-3x Virbank City Gym (only with Hypnotoxic Laser)
Hope this helps. Have fun!