Great to see a returning player of the TCG! Perhaps the first thing that you need to know is that there are two formats you could play in during the current 2016 rotation - Expanded or Standard. The former uses BW-onwards, while Standard is limited to XY-onwards. The exact sets usable in each rotation can be found
here. As both a casual player, you probably won't mind, and as a competitive player, you will generally end up participating in both formats anyway.
Here is a list of kits that are cut into into two 30-card decks (Trainer Kits) or 60-card decks (Battle Arena Decks), split from worst to best from a competitive player's perspective. The Battle Arena Decks are notably superior in content, containing many competitive cards that are also fun in a casual game. Each kit does contain a helpful guide, and you can also find the most recent rulebook
here.
XY Trainer Kit: Latias & Latios
Black & White Trainer Kit *
XY Trainer Kit: Bisharp & Wigglytuff
XY Trainer Kit
Battle Arena Decks: Xerneas vs Yveltal
Battle Arena Decks: Mewtwo-EX vs Darkrai-EX *
* Usable in Expanded only.