Dominion is a game about buying cards to put in your deck. And while treasure and victory cards may be important, the meat of the game is all about the different action cards you can buy. Here are a few basic strategy tips to help you figure out what actions to buy.
Don't Buy Too Many Actions
This is the most common mistake in most people's first few games of Dominion. Your first turn you pick up a Workshop, because a free card every turn sounds cool. Then you buy a Smithy, because drawing cards is always good. Then you workshop a second Smithy, and purchase a Cellar, and a Militia, and a Woodcutter, and before you know it, your deck is filled with actions.
Unfortunately, you'll likely find that this deck full of actions doesn't work as well as you'd like. Chances are, your hand is full of a few action cards, of which you can only play one. Worse yet, in spite of all the awesome cards in your deck, you never seem to be able to afford the very expensive stuff.
Too many actions will ruin your deck. Since you only get to play one action per turn, a hand full of unusable actions does you no good.
Aim For One in Five Actions
Try to add just enough actions to your deck to get a one action per five cards. Ideally, you'd draw exactly one action card every single turn. As mentioned above, if you draw many action cards, all but one of them is wasted. Conversely, if you don't draw any actions, then your one free action is going to waste.
You want to play your one action every turn, without having any dead cards in your hand. So, by the time your deck hits 20 cards, four actions isn't a bad number to aim for.
Your Problem Is Terminal
An important thing to keep in mind is that the above ratio only applies to terminal actions. What's a terminal action? Any action card that does not give you +actions. Cards like Market or Cellar give +1 Action, which means that after you play them, you can still play another action.
These non-terminal actions should not be counted against your one in five ratio. In other words, a good 20-card deck could have a Cellar in addition to four terminal actions like Militia or Workshop.
Cycling Cards Are Free
Actions that offer +1 action and +1 card are often referred to as "cycling", because they help you cycle through your deck faster. Such cards, regardless of what else they do, take up no space in your hand or deck. After you play one, you gain an action to replace the one you spent playing it, and you draw a new card to replace it.
Cycling cards may not always be the best purchase, but you can rely on them not to hurt your deck. In fact, if your deck consisted only of cycling cards, you'd get to play your entire deck every turn!
Don't Draw Dead
There is one case in which the above two points do not hold. 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.
Drawing action cards you can no longer play is referred to as a "dead draw". If your deck has a number of terminal card-drawing actions, you should either avoid buying too many cycling cards, or consider some action multipliers.
In the base game of Dominion, there are two action multipliers: Village and Festival. Each of these cards gives +2 actions, not only replacing the action needed to play them, but giving you an extra one as well, allowing you to play two more actions this turn.
This allows you to play a card like Smithy to draw more cards, and then play one of the actions you drew. A deck that combines multiplied actions with strong card-drawing like Smithy can be very powerful.
However, keep in mind that all purchases have an opportunity cost. Village is an inexpensive cycling action multiplier, making it an attractive card to smooth out any deck. But buying two Villages at the beginning of the game does not improve your deck at all, while buying two Silver for the same price will rapidly increase your buying power.