First off, welcome to PokéBeach! I hope you enjoy yourself here.
Anyways, to answer your question, the best starter deck may depend on whether or not you're looking at getting into competitive or casual/just-for-fun play.
For competitive play, the first thing you need to know is that the TCG is now split up into two competitive formats - Standard, which is XY-on, and Expanded, which is BW-on. As a competitive player, you'll probably end up participating in both formats.
Here is an official list of the legal expansions usable in each format, which provides more details on the Standard and Expanded format.
Power levels and some mechanics will and may have changed depending on when you stopped playing. To get your head around the changes, I would definitely begin with a starter deck or another pre-constructed deck. Theme Decks are you cheaper although not very competitive options. For Standard Theme Decks, I would recommend Destruction Rush or Resilient Life from a competitive standpoint due to them containing some major players in competitive decks, Yveltal and Xerneas/Aromatisse respectively. For a more recent and easier-to-obtain deck, you may like to try Night Striker from BREAKThrough or Wave Slasher from the soon-to-be-released BREAKPoint, which contain some competitive/potentially competitive cards like Zoroark and Greninja respectively. In Expanded, there is also Shadows and Furious Knights Theme Decks, which are quite strong in their own right, however both are older decks and would be difficult to find.
However, there are other preconstructed decks that you may prefer to obtain, due to their better value (they contain many staples) and better simulation of competitive/powerful decks. These would be the
Xerneas vs Yveltal Battle Arena Deck and the
Mewtwo vs Darkrai Battle Arena Deck. I personally recommend the latter, as it contains more useful staples and will also give you the taste of powerful Pokemon-EX, a mechanic popular in the current format. These will, of course, cost a bit extra, and can be difficult to find, but they are really worth your money and is my first recommendation for both old and new players.
If you simply just want to obtain something for fun, you may want to identify a Pokemon or type that you particularly like, and make your deck choice from there.
