You chose to lead the 8 of Diamonds. This is the correct leadas it is the fourth card down from the top in your longest suit, which is Diamonds. Well done.
Now the Dummy is revealed.
WEST -lead 8 Diamonds
As soon as the dummy is revealed, you, the Declarer should take time to look at the two hands and plan your strategy.
You know that Diamonds are likely to be West's long and strong suit, because of the lead, but the fact that he led the 8 is unusual. Usually the 4th highest card is a small card. That means that West will have 3 higher cards in his hand, and between you and the dummy you have the Jack, Queen and King - so I would assume that West had the Ace, ten and nine and possibly another smaller diamond in his hand. So you could win the trick in either of your hands. Usually if you have that choice, you choose to win in the hand holding the smaller number of cards in the suit, or in the 4th hand, if as in this case, they are equal in length. So you will play the 6 of Diamonds from Dummy.
Below we have East's hand
Remember that your partner played the 8 of Diamonds. Then Dummy played the 6 of Diamonds. You however cannot go any higher, so you would play your lowest card, which is the 2 of Diamonds. South would win the first trick with the King. But East would remember that his partner had an interest in Diamonds, and when he gets in later, will play that suit back to him.
Declarer now has to decide how to continue to play the
game, in order to make 7 more tricks. He counts his winning cards - Ace
and King of Clubs, and Ace of Spades. The King of Diamonds has already
won and the Jack or Queen will win later and the King of Hearts (after
the Ace has been played). But there are still two more tricks to be won.
The best option is to try to get a long suit ESTABLISHED.
This means that you play all the high cards in the suit and then your low
cards become winners. Unfortunately, you do not have any really long suits,
so your best combinations, having 7 cards in each, are Spades and Hearts,
but of the two, Hearts looks marginally better having slightly higher values.
Therefore your strategy is to try to establish the fourth Heart in your
hand for an extra trick.
But you don't have the Ace of Hearts, so again your strategy in this hand will be to LOSE YOUR LOSER'S FIRST. Your 8th trick is not at this stage obvious. So you know that this is going to be a difficult contract to make.
When you are playing in No Trump, the thing you do not do, is play out all your Aces and King winners straight away. You plan to establish your suits, losing whatever cards needed in order to do that, before you cash your winners. To do it the other way around, would most certainly signal disaster for you.
17. What card will you lead to the next trick?
a. 2 of Hearts
b. 2 of Clubs
c. 4 of Spades