Let's give you some more specific advice, eh?
Such as what T/S/S cards are considered staples, what the metagame is looking like right now, which sets are the best to invest in and what cards are viable.
Currently, deck engines will generally consist of 4 Professor Sycamore, 2 to 3 N (or 2 Shauna/Birch in Standard), 3 to 4 VS Seeker and 4 Ultra Ball. Most decks reaching top tables will be running Shaymin-EX to keep up with speed and consistency, but unfortunately the card itself is quite expensive. Decks often also contain four or so copies of Puzzle of Time or Trainer's Mail if it has space. Aside from draw supporters, decks often will include 1 or 2 Lysandre and 1 or 2 Hex Maniac, with Delinquent, Brigette and Pokemon Fan Club other commonly used supporters. Of course, it's better to experience and analyse decks yourself, so you may like to check out our
Metagame Deck Discussion or some of the top decks (
Standard,
Expanded) of this year's professional play.
For checking out meta, there are a few resources you can use. Of course, looking through winning decks is one option. Another is to check out our
front page articles, some of which have some excellent summaries of the meta-at-point-of-writing. The official Pokemon website has also recently written a summary of the meta, which you can check out
here. Although I'm not fully in tune with this year's meta, I am aware that the
Night March variant has recently become a big contender - with over half of the top tables of States being Night March, last time I heard.
Vespiquen/Vileplume variants have came to light to counter this, along with
Greninja BREAK* and
Trevenant BREAK variants that also have better matchups.
Yveltal/Zoroark/Gallade variants were also contenders, but I'm not sure how well tiered they are with Night March running rampant.
*BREAK are a very new mechanic that are very similar to the old Lv.Xs
Set investment is a funny one. You'll find that Phantom Forces is a very solid set, with not only containing the Night March family, but also Dimension Valley and Battle Compressor vital for Night March (and good for other decks), and staples such as Sycamore, Shauna and VS Seeker. It should be noted that Phantom Forces is at risk of being rotated for next season, however a recent Japanese set reveal appears to be a reprint set (reprinting major players of Phantom Forces), which we could easily see in English. Roaring Skies may also be a good investment. The Shaymin-EX ideal in virtually every deck costs insane money, even around the $40s. The EX can be found in Roaring Skies, but you would have to be quite lucky in pulling it. Roaring Skies also has Trainer's Mail, which would undoubtly be nice. Aside from those, you may like to look at Ancient Origins if you are looking at running Vespiquen/Vileplume, or BREAKPoint for Greninja BREAK.
Also have a question along with the whole Expanded/Standard format. How does that work exactly? And what should I prioritize?
RuskeD's explanation isn't half bad, but I'll give my two cents too, since I'm here.
Standard and Expanded are the two major formats in professional play. This year, Standard consists of all cards XY-on while Expanded has a much broader cardpool starting from BW-on. If you're looking at getting into professional play, you're probably going to need to play in both formats. For the 2016 series, I'd use
this information to give you an idea of the importance of the Standard and Expanded. As you can see, the two formats are used relatively equally.
