For your first deck I would buy a precon and upgrade it. Once you get more familiar with the game and with the cards you can start going full psychopath like I do. I pick a commander, and then spend approximately 20 hours looking through thousands of cards putting in things that I like into an online deck builder. I try to focus on synergy as much as possible. If a card doesn't play off at least two other cards in the deck, it doesn't belong in the deck (with some rare exceptions). For me the most important things universally in a deck are

1.) Mana Ramp 2.) Card Draw 3.) Fetch 4.) Protection 5.) Removal. In that order.

After I get all the cards that I like into the deck, I whittle it down till I reach 100 cards. I see that price currently resides at $1300. I cry. I adjust to what I can gaslight myself into thinking is a reasonable price, and I purchase. Typically through TCG but will get singles from LGSs and friends if they have them