Lesson 4: Responding

As responder, with a fit for opener’s suit (3+ cards), bid like this:
6-9 ponts. Raise partner's opening to 2 (1♠ P 2♠)
10-12 points. Raise to 3(1♠ P 3♠)
13+ points. Raise to game (1♠ P 4♠)

These bids are called Limit Bids, as they limit the points in the hand to within a certain range. When responder knows there’s a fit, their response may include any shortage points (in other suits), as these improve the trumping potential of the hand.
For each doubleton (2 cards in a side suit) add 1 point; each singleton (1 card in a side suit) add 3 points; and each void (0 cards in a side suit), add 5 points. Add shortage points only when a fit has been found. 

When there is no immediate fit, bid your own suit if you have 6+ points. Any new suit by responder is forcing, (opener must bid again), as the partnership is still trying to find a fit. If responder bids at the next level, in their own suit, because their suit is ranked below opener's, they show a stronger hand.(e.g. 1♠ p 2♦ )
Sometimes responder has 6 – 9 points, and none of the above. The recommended response is 1NT, a limit bid.

Summary:

Minimum Hands
With 6-9 points, raise to two of suit opened (1♠ p 2♠) with  a three+ card fit
6+points, bid a new suit at the one-level (1♦ p 1♥) with your own suit
6-9 points, bid 1NT(1♠ p 1NT) with no fit (shows few than three spades)
 

Medium Hands
10-12 points, raise to three of suit (1♠ p 3♠) with 4 card fit
10+ points, Bid  a new suit at the two-level (1♠ p 2♣)
 

Maximum Hands
13+ points,  raise to game (1♠ p 4♠) with a fit
13+ bid 2NT or 3NT (1♠ p 2NT or 3NT) to show a balanced hand
13+ bid your own suit at the two level. (1♠ p 2(♣/♦/♥) with four+ cards in your own suit).