For any contract, when dummy comes down, "MAKE A PLAN." (Bridge teachers and writers are required by law to say/write that). But, what plan?
It will help if you can get in a habit of doing/thinking the same thing on every deal. In notrump, the thinking is very different from in suits. That is the subject of another article.
For suit play, this is what I recommend:
1) Go through each suit in your hand (or, if dummy has the long trumps, go through that hand instead).
2) As you look at each suit, notice what help you have in the hand opposite and decide how many tricks you will LOSE. For example, you are in 4, and see that you have A32 in your hand. The dummy has 1076. Count that as 2 tricks you will lose. If dummy has AQ and you have 65, say to yourself: "Maybe 1 trick to lose--if the finesse is wrong."
3) Figure out what you can do with those losers. You are in 4 and have A32 in your hand. Dummy has 76. If dummy has enough trumps, you can plan to ruff a heart in dummy. So, count on winning one heart, losing one heart and ruffing one in dummy. If dummy has a running suit (or one that can be set up), plan on possibly throwing heart loser(s) on diamonds. There are basically 3 things you can do with losers: a) trump them in dummy, b) discard them, c) finesse. Decide if you must try to get rid of the losers right away ("fast" losers), or if you have time to get rid of them later ("slow" losers).
4) Decide if you will draw trump at once -- or postpone drawing trump. General rule: If there is no reason not to draw trump, then draw them. You don't have to memorize the reasons "not to draw," but here is a brief list of the most common reasons not to draw:
a) You need to ruff something in dummy.
b) You need trump entries to your hand or dummy.
c) You need to first take (or establish) a discard for a loser(s).