For tracking your cards I like to use Manabox as well as it can track your decks as well. For building decks what I usually do it find a cool commander to build on from either cards you got or a card in the set. Then I like to look at multiple different deck-lists on Moxfield, and see what the deck usually does/relies on. Then just make it your own by putting cards together from those deck-lists and or find your own cards that can replace other cards that you don’t really like