Fundamentals of Modern Competitive Bidding

When the other side has opened the bidding, you should try to make a call if possible. An overcall shows a decent 5+ card suit, and at the one-level, 9 – 17 points e.g. 1♦ (1♠) BUT, the two level (you have clubs and they open 1♥), requires 13+.
An overcall is not forcing, but partner should try to raise with a fit, almost regardless of points.

A double is a bid which occurs when the opponents have already opened, and suggest your side should make a contract. “Double” (a “X”), takes no bidding space, but shows an opening hand, with shortage in the suit opened, and able to support the other three suits.
The doubler’s partner will bid their longest suit, and show their strength at the same time. Bid at the lowest possible level with 0 -8 points, jump one level if 9 – 11, and jump to game with 12 +points. If they have the choice of a major or a minor suit, prefer the major.