The three purposes of bidding are to describe your hand, find the fit AND agree the contract.
An opening hand will normally have at least 12 points.
In ACOL we can open with a four card major but it is preferable to open a five card suit.
Many hands are unbalanced and have one suit longer than the other. Where there is a 5 card and a 4 card suit, bid the longest first. On your second bid you will be able to show your second suit and your partner will know which is the longer suit.
If you have two five card suits bid the senior one first.
If you have 4 spades and 4 hearts bid hearts first to allow your partner the chance to bid spades at the lowest level.
Where you have a hand with a lot of cards in one or two suits but less than 12 points you can use the Rule of Twenty. Add the points in your hand to the number of cards in your two longest suits, if the total is twenty or more you may open your longest suit.
Where your hand is strong (16+) and has a long suit you can open at the 2 level. This is called Strong Twos.
Where you have a weak hand (6 to 9 points) and a 7 card suit and you are not vulnerable you can open at the 3 level. This is called a Pre-emptive Bid. This has the effect of making it difficult for your opponents to find their contract. If this opening is used the opener must not bid again.
With balanced (flat) hands with 12 to 14 points bid 1NT. This is a precise description of your hand so do not deviate from the rule. 11 or 15 points do not merit 1NT.
With a balanced hand with no five card suits and more than 14 points bid your best four card suit. If you bid again you can bid no trumps to show your partner a strong flat hand.
If you hand is balanced and has 20 to 22 points bid 2NT.
For hands with 23 points or more bid 2C. The response to this bid is always 2D so that the opener can bid again. An opening of 2C is game forcing. This means bidding must continue until a game contract is found. The only exception is if the opener rebids 2NT which shows a flat hand. In this case if the responder has 2 points or less the bid can be passed.