When holding 13 to 21 points you should open the bidding.
You MAY open with only 12 points if you have a good suit and good middle cards.

OPENING BID PRIORITIES
1. 13 - 21 points and a 5-card or longer Major suit, open 1♠ or 1♥
2. 15 - 17 points and balanced distribution (4-3-3-3 or 4-4-3-2 or 5-3-3-2) open 1NT
3. 20 - 21 points and balanced distribution (4-3-3-3 or 4-4-3-2 or 5-3-3-2) open 2NT
4. 13 - 21 points and none of the above, open 1♦ or 1♣

WHICH MAJOR SUIT TO OPEN
• when the suits are different length open 1 of the longer suit
• when the suits are the same length open 1♠, the higher ranking suit

WHICH MINOR SUIT TO OPEN
• when the suits are different length open 1 of the longer suit
• when the suits are the same length open 1♦, the higher ranking suit
• exception to the above - when you have three of each open 1♣

The general rule to follow is that you open a longer suit rather than a shorter one, and the higher-ranking one when the two suits are equal length.