The Opponents Open the Bidding
The side that takes the first shot usually has the advantage, but when the opposition is well armed, it often doesn't matter who gets in the first blow.
When one of your opponents opens the bidding, you have some choices. You can:
Pass. This tells your partner you have nothing to say at the present time.
Overcall in a suit — as in bidding 1 ♠ after your right-hand opponent starts with 1 ♥. Your 1 ♠ bid is an overcall. Making this call generally shows a suit of five or more cards of decent quality. The overall strength of the hand should be appropriate to the level of the bid and the vulnerability.
Overcall in no-trump, usually either 1NT (showing a balanced hand, a stopper in the suit that was opened, plus 15–18 high-card points) or 2NT (a conventional bid showing at least five cards in each of the two lowest unbid suits).
Make a cuebid of opener's suit (as in 1 ♣ — 2 ♣). This is another conventional way to show two-suited hands.
Make a takeout double. A direct double of an opening bid tells partner that you have the rough equivalent of an opening hand and at least three-card support for the unbid suits. Partner is not allowed to pass unless he has a long, strong holding in opener's suit.
As you can see, there are many options available for competing. Experience will teach you the right one for each different situation.
Roads Not Taken
Astute players use information about options their partners could have taken to draw inferences as the bidding and play progress. You can make deductions about your partner's hand from her failure to take some action that was available to her.
Listening to the bidding is more than just hearing the calls. Really listening means interpreting the messages and using them to your advantage.
It is usually safer to take action in the direct seat — that is, next to bid after an opponent opens — than when both opponents have bid and exchanged information. The more they know about each other's hands, the better they will handle your interference.
