Don't sit back and let the opponents bid all over you. Try to get into the auction - if possible. One of the most useful 
competitive tools is the Overcall - bidding over  the opponent's bid(s). A bid of 1 D by your opponent followed by a 1 S bid by you can be done with less than 13 points. (An Overcall at the one level can be done with as little as 10 points.) Your Spade suit should have 5 or more cards; and the suit should be headed with 2 honors. Why 2 honors? Since the opponents did open the bidding, there is a chance that your side might not get the contract. And if the opponents do get the contract, your partner will most likely lead the suit that you bid. So have 2 or more honors.  Lead direction is one of the advantages of overcalling,
      To make an Overcall at the 2 level, you should have 12 points with a five card suit headed by 2 honors. To Overcall in No Trump you should have a 1 No Trump opening  bid: 16 - 18 High Card Points (HCP)and a balanced hand, plus one more special feature. And that feature is a stopper in the opponent's bid suit or suits. Example: your right hand 
opponent bids 1 H. You have 17 HCP's and a balanced hand. To overcall 1 No Trump you need a stopper in  Hearts. That is one or more high honors or two or three low honors in Hearts.
      A pre-emptive overcall is made by making the  pre- emtive bid that you would normally make if you were the opener. A 1 C by your right hand opponent followed by a 3 S bid by you, shows that you have a 7 card Spade suit with less than 10 points.  A natural 3 S pre-emtive opening bid by you has just become a pre-emptive jump overcall.
      These three types of Overcalls will be discussed in more detail in the next book. Other competitive tools and tactics will also be considered.  Competition is what makes Bridge an exhilarating and challenging game.  Remember Ely Culbertson's last words: to bid is human, to compete divine.