I think some important core ideas to understand:
1) Recognize cantrips
Action cards that give at least +1 Card and +1 Action are called by the community "cantrips." Because playing an Action costs 1 card and 1 action, a cantrip is a card that functionally replaces itself in your hand. A card that reads "+1 Card and +1 Action" is practically like a thing that isn't in your deck. Village and Merchant are two of the earlier cantrips you'll see in the game. Note that Village gives you 1 additional +Action and Merchant can give you +$1 if you meet a condition. Both cards are useless if you aren't able to leverage the additional effect that they give you.
Your starting deck has about a 1/6 chance of producing $5 on a turn, so if you are only buying useless cantrips in your deck, that chance isn't going up. The average game of Dominion lasts around 15 turns, so waiting to roll a 1 in 6 to buy higher-cost cards is really bad. Playing 5 Villages and no Actions that actually use the +Actions you've accrued means you've wasted time buying so many Villages that you haven't gotten to use. You should think about what your cards are going to do for you the first time you draw them, as there is usually enough time to acquire what you eventually need later.
2) Appreciate terminal space
Cards like Council Room and Library and Mine are what we call "terminal Actions" because they do not give back the +Action it costs to play them. Regardless of how much each Action in the game costs to buy, every Action costs 1 +Action to play. Because you can only play 1 Action each turn without something giving you +Actions, you should carefully consider where that 1 Action is going to go. If cards like Village or Festival or Throne Room appear, you can play many terminal Action cards in a turn, but if you only have +1 Action cards like Cellar or Market or Harbinger, you can only play 1 terminal Action and so should choose that terminal Action carefully.
If you buy too many terminal Actions based on the +Actions in your deck, you'll find yourself drawing cards you can't play: Which means not only did you lose time buying that card, you've also lost the opportunity to play it that shuffle (may as well have bought a Curse!).
3) Don't undervalue economy
If there is an important, powerful $5-cost card in the Kingdom like Witch or Sentry, one of the first two cards you buy should probably be something that produces at least +$2, like Silver or Militia or Moneylender. As stated previously, your starting deck has only about 1/6 chance of getting a $5-cost card, so a source of $ as described kick starts your economy and gives you a very good chance of buying a $5-cost card on turn 3 or turn 4.
On a mathematical way of thinking, you can imagine +Cards in terms of +$ value. This is a concept we call money-density: Your starting deck has $7 across 10 cards, making your money-density $0.7 per card. Therefore, drawing 2 cards from your starting deck (like with Moat) expects to produce $1.4, while drawing 3 cards (like with Smithy) expects to produce $2.1. If you add a Silver to your deck, you will expect $1.6 and $2.4 respectively. If you added another Action like Workshop to your deck, you would actually reduce your money-density ($7 across 11 cards (excluding the drawing card)), expecting $1.2 and $1.8.
4) Don't overvalue stop-cards
Any card that does not allow you to draw more cards is called a stop-card. Coppers, Estates, Curses, and Provinces are all stop-cards. The more stop-cards you have in your deck, the more likely your 5-card hand will stop before you get to all the cards in your deck, where non-terminally increasing your hand size (such as with Laboratory or a combination of +Action cards like Village and +Card cards like Smithy) lets your deck theoretically function with more stop-cards (though less consistently0. Because the cards you add to your deck tend to be better over time, seeing all of them every turn is ideal (until you start buying Provinces).
This is primarily what makes Copper and Estate such bad cards and cards that give out Curses like Witch so powerful: If you're going to be stopping one of the 5-cards you get each turn, you may as well get something better than +$1 or even nothing in the case of Curse and Victory cards. The weakness of those cards is why trashing them is so, so good. If your deck has only 4-stop cards in it, you will see every card in your deck every turn (assuming you haven't bought more terminal Actions than you have +Actions to play).
If you're a step ahead of me, you may notice that Silver, Militia, Festival, and even Gold are also stop-cards. Interesting, that.