What format are you trying to play? What are the most influential decks in said format? What is the general strategy you're trying to accomplish? Does your strategy beat the best decks in the format? These are all question you have to ask when creating a deck. I would suggest when you are first learning to play, pick a format first. I would suggest an eternal format, Vintage, Legacy, or Vintage. Standard is lowest cost of entry, but cards rotate and that $300 you spent on that sick standard deck rotates out and now that's $300 deck is worth about $2 dollars. The eternal formats have a higher initial price of entry, but cards never rotate out and once you get your deck you don't need to buy any more cards unless you want to or you want to make changes to your deck.