One of the most confusing aspects about learning bridge is that not only are there lots of rules to learn, but different rules for different situations!

Opening the Bidding
You can only open the bidding if nobody else has yet bid!  (A 'pass' does not count as a bid)
If somebody else has bid, then you need to look at a different section...

As a general rule, you need 12 High Card Points to open the bidding.  Even with as much as 19 HCP, you should open the bidding at the 1-level only.  We will look at higher-level openings on another date.

Overcalling
This is the term for bidding when your opposition has already opened the bidding.   It does not matter if you could have opened the bidding (if you had had the opportunity) - different rules apply to bidding in this situation.  We will cover these in our next few lessons.

Responding
This is what we call bidding when partner has opened the bidding.  Again, it doesn't matter if you could have opened the bidding - you have to follow a different set of rules.

Basically, if partner has opened the bidding with 1 of a suit, you can - and must - respond if you have as little as 6 High Card Points.  Responding to 1NT is entirely different, of course!

Rebidding
Your second bid in the auction is know as your Rebid.  When you open the bidding with 1 of a suit, that gives partner a fairly vague message, but it is your Rebid that really describes your hand - often to an extent that partner (Responder) can make a decision about the final contract there and then.

Even when you open the bidding, you should be able to plan ahead to what your second rebid will be.  Of course, you can change your mind, if partner does something unexpected, but assuming that he responds by bidding a new suit...

Don't just think about what to open, think in advance about what you will rebid (what you will bid on your second turn).
When partner responds in a new suit, you must bid again, so you may as well be prepared:

1. with a balanced hand

12 - 14 HCP   Open 1NT
15 - 16 HCP   Open 1 of a suit, rebid NT at the lowest available level.
17 - 18 HCP   Open 1 of a suit, rebid NT, but skip a level.
19 HCP          Open 1 of a suit, rebid 3NT!

2. with an unbalanced hand - only one long suit

Open your long suit, then...
12 - 15 HCP   Rebid the suit at the 2-level.
16 - 18 HCP   Rebid it at the 3-level.
19 HCP          Jump to the 4-level
(if it is a major suit - consider other options if it is a minor)

2. with an unbalanced hand - two long suits

Open your longest suit (which will be at least 5-cards in length), then...
12 - 15 HCP   Bid your second suit, only if not a reverse.  (see 29th January)
   If you cannot bid your second suit (without going past the barrier), rebid your first suit at the 2-level, as if it was your only suit.
16 - 18 HCP   Bid your second suit, whether or not this constitutes a reverse.
19 HCP         Bid your second suit as a reverse (at the 2-level if it is higher-ranking, at the 3-level if it is a lower-ranking suit).

Whenever you have 6 HCP or more, you must make a response, just in case partner has as many as 19 HCP.

1.  If you like partner's choice of suit (especially if it is a major suit), show it by bidding the suit.
2.  Otherwise bid your own 4-card suit if you can.  A new suit bid forces partner to bid again, describing his hand more accurately.  Even with a strong hand, there's no need to bid the suit any higher than the lowest available level.  But with a weak hand (less than 10 HCP) you are not strong enough to bid a suit at the 2-level.
3.  Bid No Trumps, or support partner's minor suit, as a last resort.

6 - 9 HCP:  (game is possible but unlikely)
   Support partner's suit at the 2-level;
   Bid a suit at the 1-level if you can;
   otherwise bid 1NT.  Do not bid a new suit at the 2-level.
10 - 12 HCP:  (game is quite likely, but not certain)
   Support partner's suit at the 3-level;
   Bid a suit at the 1-level if you can;
   otherwise bid your longest suit at the 2-level;
   bid 2NT if you must, but prefer a suit response if possible.
13 - 15 HCP:  (you definitely have enough points for a game)
   Support partner's suit at the 4-level;
   As above, bid a suit at the 1-level if you can;
   bid 3NT if you must, but prefer a suit response if possible.
16 + HCP:  (easily enough for game; maybe you want to explore a slam!)
   If you have a very good suit, make a jump bid in the suit
   otherwise, as above, bid a suit at the lowest available level, to find out more about partner's hand.