Cutting to the chase:
When getting into Magic the Gathering, it's easy to get overwhelmed and confused by choice. Instead of thinking about cards, I've learnt to think about the game mechanic roles that a deck needs to have satisfied, and then I look for cards that fill those roles.
My list is by no means definitive, but I think these are the basic universal roles you need, regardless of how you play:
- Mana base: Land cards. These provide you with mana so you can play cards. For a 60 card deck, anticipate requiring about 24. In a later episode, I'll talk about additional ways to manage mana.
- General purpose: Creatures, enchantments, instants, artifacts, and so on. These are cards, costing a full spectrum of mana amounts (some you can play for 1 or 2 mana, others require 4 and 5 and more), that you need to populate your deck. These don't need to be anything special, unless you want to spend money for really fancy cards, but I assume you're not listening to me if you're a pro player./i>
- Card draw: You're going to deplete your hand unless you have cards that give you permission to draw more cards than just your allotted one-per-turn.
- Recursion: When you're forced to discard a card you really really wanted to play, you'll be glad to have some cards that grant you permission to bring that card back from your graveyard. This is called "recursion" by Magic players.
There are other card types that can be useful, but they may be best for specific strategies. The ones I list here are, I believe, pretty universal.
You can find lists of cards that fill specific roles with a simple Internet search. For instance, if you realize you have no cards that let you replenish your hand, you might search for "best cards to draw cards MTG" and get directed to a site like TappedOut.net or magic.wizards.com.
Go to your local game store or an online vendor to buy just the cards you need. Because there are so many to choose from in any given category, you get to control the price. I have a rule for myself that do not spend over $2 on any single card.
When trying to fill roles, I generally aim to get 4 cards that do the thing I need. Sometimes I get exactly the same card 4 times, other times I get a variety of cards to fill the role. Either way, 4 usually feels like a good draw frequency for each role.