Opener: the first player to make a bid
Responder: the opener's partner
Overcaller: the player to the left of the opener
Advancer: the overcaller's partner
12-14 points, balanced hand.
Pre-alert. If your partner opens 1NT you announce to the table "12-14"
Responding to 1NT
INT is such an accurate bid that the responder will be in a good position to choose the final contract.
There's normally no need for the 1NT opener to bid again.
Opening 1♣, 1♦, 1♥, 1♠
12+ points, 4+ card suit.
Pre-alert: If your partner opens 1♣ you announce to the table "4+"
Responding to 1♣, 1♦, 1♥, 1♠
You can respond with 6+ points to show trump support, bid notrumps or bid a suit of your own. If you bid your own suit the opener should bid again.
The overcaller and the advancer can use a similar bidding system to the opener and responder. The overcaller can be more flexible with points but it's better to have a 5+ card suit to overcall.
When everybody is bidding, use your trump fit to guide you how high to bid. With a trump fit you can be reasonably adventurous even at the 2 or 3 level. Don't go too crazy higher than that and if you don't have a trump fit then you'll often be better off defending.
If it's just you and your partner bidding then try for game with a combined point count of 25+.
Your contract is not a target. Your target is winning as many tricks as you can.