Which Suit To Respond

If partner has opened with a bid of 1 of a suit and you have fewer than 12 points it is usual to respond with a bid at the one level if you hold a four card major suit that you can bid, even if you hold a longer minor suit.  This is because you will probably only have a strong enough hand to bid once and it is important to show your 4 card major holding

If you do hold 12+ points then you can bid your longer, minor suit first as you have enough strength to bid again.  This will give you an opportunity to show your partner that you have 5-4 shape (at least).  As a bid of second suit is a forcing bid, forcing partner to bid again, it is likely that the bidding will finish at the three level or higher (especially if your second bid at the 2 level was in spades).  This is why it is important that you hold 12+ points if you bid a longer, minor suit first when you also hold a 4 card major.

Here is an example
You are South and you hold:
Spades: Q J 4 3
Hearts: 8
Diamonds: K J  8 5
Clubs: K Q J 9 3

The bidding has gone as follows
North   East   South   West
1 H            Pass    2 C          Pass
2 H           Pass     2 S
With this hand it is right for us to bid  2C as our first responding bid (rather than bidding 1S) as we have 12+ points and so have enough strength to bid again.
When North bids 2 H, we can bid 2S. This shows partner than we hold 12+ points.  Our bid of a second suit is forcing and shows partner that we hold at least 5 clubs and at least 4 spades.  Given this knowledge partner can make an appropriate rebid.