by Thanee
You should start small and not try to build a deck that can handle all scenarios. At first try to get a deck together to defeat one specific scenario, looking at its specific dangers and what cards help to overcome them.For example, in the Anduin scenario you might want not too much threat initially from your three heroes, to have a few rounds before engaging the troll, but also good questing and fighting power. Also, cards to help against locations are good, like the Northern Tracker.
And you should start with building 30 card decks (the size of the preconstructed decks), which is much easier than building 50 card decks, especially since you would need almost all cards from two spheres to get those 50 cards together with just a single Core Set as your card base.
Additionally, until you feel more secure, you should use the option mentioned in the rulebook and not draw cards for shadow effects at all.
Another simple thing to keep in mind: dont play cards just because you can. Use them in situations where you really need them.
For example, the Lore card Secret Paths (one location in the stating area does not count for the quest difficulty) can be nice to add some extra progress tokens on the adventure in one round, but it can also be absolutely vital to have that -5 location not count in another round, and if you have used it before in a situation where it was "only nice", you won't have it in the situation where it is "absolutely vital".
Bye
Thanee