By following a simple cascade of reasoning, you can determine whether and what to open. While you can start with either the weaker or stronger hands, starting the checklist with the stronger hands makes it easier because you can eliminate the confusion over borderline hands. Work your way down the list of question below, stopping when you answer one in the affirmative:

Do you have 22+ HCP or the ability to win 9 quick tricks if you pick the denomination (in this second case, you should still have 17+ HCP)? If yes, open 2C.

Do you have 20-21 HCP and NT distribution? If yes, open 2NT.

Do you have 18-19 HCP and roughly NT distribution? If yes, bid your suit at the 1 level and then jump-bid in NT (skip one available NT level).

Do you have 15-17 HCP and NT distribution? If yes, bid 1NT.

Do you have a Rule of 20 opening (rule of 18 or Pearson Points in 3rd or 4th seat respectively)? If yes, bid your suit at the 1 level (note that a hand with few HCP and a long suit should probably be reserved for a preempt unless you have a respectable honor holding in it).

Do you have a 6-card suit and 5-11 HCP? If yes, bid a weak 2 (for Clubs, open at the one level on Rule of 20 or the 3 level for a solid Club suit: 2C is a reserved bid).

Do you have a 7-card suit and 0-9 HCP? If yes, bid a weak 3.

Do you have an 8-card or longer suit? Bid at the 4 level (note that you may have up to 14 or 15 HCP with this length).

If you don't meet any of those requirements, don't open.

Always keep in mind that you are trying to give your partner an accurate picture of your hand. Furthermore, with the exception of preemptive bids, do not open unless you are prepared to rebid over any non-preemptive overcall your opponents may make. This allows your partner to derive certain key information from what you say.