by GeckoTH
For starters, you can take the insert in one of the core sets and invert the folds on it, giving you two troughs on either side instead of one down the middle. If your cards are unsleeved, this should give you lots of room. You can also grab a set of dividers I made here, and you'll end up with something like this:
If you happen to sleeve your cards, you'll run out of room with all those Adventure Packs. You might need to use both core boxes or some other storage container at that point.
As far as deckbuilding goes, head on over to cardgamedb.com (or the strategy section of this forum), and you should find plenty of submitted decks. Pick a couple based just on what Heroes or Spheres appeal to you and play around with them. You'll eventually become familiar with the various cards, and how they can work together.
If you really want to go more in depth, I'd recommend checking out the Cardboard of the Rings podcast. Each episode clocks in around 1-2 hours, so it can be a little daunting to jump into. But each episode typically dives into an Adventure Pack, talking about each card, how useful they think it is, and how it combines with everything else. Before you know it, you'll be able to recite cards from memory, and know how to best utilize them.