In Dominion, it is important which cards you purchase for your deck.
While treasure and victory cards are very important, the action cards will make the most difference.
Let's give you a few tactical tips to help you to figure out what action cards you should buy.
Don't Purchase too many actions
Many players make this mistake in the beginning. Before you know it, your deck is full of actions and this doesn't work as good as you will expect.
Chances are that your hand is full of a few action cards of which you can only play one.
Even worse, without good treasury cards, you will never be able to afford the expensive cards.
Try to have 20% of your cards action cards
The perfect situation would be that every set of 5 cards hold one good action card.
When we have more action cards, we will not able to use all of them, when we have less action cards, we can not execute an action each turn.
Your Problem is Terminal
The 20% ratio is not strict. Keep in mind that there are action cards that gives you extra actions. e.g. Cards like Market or Cellar give +1 Action.
Such cards should not be counted in your 20% ration.
Cycling Cards are Free
Actions that offer +1 action and +1 card are often referred as "cycling", because they let you cycle through your desk faster.
Such cards, don't take any space in your hand or deck.
Cycling cards are not always the best choice, but you don't damage your deck with them.
Don't Draw Dead
There is an exception to the above two points.
If you play a card like Smithy with your one action, and draw three new action cards, you have no actions left to play them even if they would normally cycle.
If your deck has a couple of terminal card-drawing actions, you should better avoid buying too many cycling cards or consider a couple of action multipliers.
There are 2 action multipliers: Village and Festival and they give +2 actions.
This allows you to play a card like Smithy to draw more cards, and then play one of the exta actions you drew.
Action Multipliers can be powerfull, but still remember that all purchases have an opportunity cost.