When making an opening bid you must be planning your second bid, called a rebid.
If you open and your partner responds by bidding a new suit you MUST bid again for one round.
With opening values and a balanced hand it is better to bid 1NT. The opener must then not rebid unless invited.
With an unbalanced hand the longest suit is opened first (length before strength).
The rebid is determined by a simple idea called the barrier. When making an opening bid, the opener must imagine a barrier at one level above the opening bid. ie.
1C has a barrier of 2C
1D has a barrier of 2D etc.
Any rebid over the barrier shows a strong hand of between 16 and 19 points. A rebid over the barrier could be the opening suit or a new suit.
1S - PASS - 2H - Pass
1S - 2D - PASS - 3D
The rebid at the 3 level is over the barrier of 2S showing a strong hand.
The rule on opening bids is length over strength meaning the junior suit might be bid first. If partner does not support your opening suit you will bid your second suit showing 5 in the first and 4 in the second. This may involve bidding at one level higher than if the senior suit was bid first.
When the senior suit is bid second at the 2 level, it is called a reverse and requires an opening hand of at least 15 points. If the opener does not have a hand strong enough to reverse, she must rebid her first suit. For example,
1H - 2D - PASS - PASS
The opener has rebid a senior suit to the opening suit. This is a reverse.
A senior suit bid second at the one level is not a reverse.
When the opener is not strong enough to bid above the barrier or reverse, she must rebid her opening suit. This is called a limit bid, showing only opening values.
Rebidding the opening suit usually promises a 6 card suit but 5 of good quality is acceptable.
The responder, knowing the strength of the openers hand can now continue bidding. A bid of 2NT by the responder closes the bidding.
Strong Balanced Hands
With a strong balanced hand the strongest 4 card suit can be opened and a rebid of no trumps will describe the strength:
with 15 to 17 rebid 1NT
with 18 points 2NT
with 19 points rebid 3NT