The main difference between duplicate bridge and regular rubber bridge is strategy.
In rubber bridge, you are scored on what you bid. Thus, if you bid two of a suit and make three, you only get credit toward game for the two tricks. Though you get points for making the extra trick, they don’t count toward game. So you want to get the contract for as high as you can make it.
In duplicate, you are scored by how many tricks you make. Thus, if you bid two and make three, you’re given credit for three. As strategy, then, you want to get the contract for the lowest level you can, since not making your bid is penalized. You also get bonus points for making game – three no trump, four in hearts and spade, and five in diamonds and clubs. Thus, the question you are always asking is “can I make game with this?” If you can’t, you stick with the lowest level that gives you a contract: two clubs is just as good as four clubs, and less risky.
As for the bidding in duplicate – you can use any system you want, including standard bidding. Experts tend to use conventional bidding to better describe their hands, but there’s no rule requiring it. It would help if you knew the conventions, but the rules of duplicate bridge allow you to ask the bidder’s partner what the bidder meant (you can’t ask the bidder). Thus, if I bid one club, you can ask my partner what I meant by that.
There are plenty of low-level duplicate bridge tournaments and games where you can stick with standard bidding.
Defense, too, is primarily an issue on how well you communicate with your partner. There are also some general rules (e.g., always lead back the suit where your partner led to your ace that he had no reason to know about) that apply to bridge at any level, not just duplicate.
Bridge in and of itself does take time to learn: you need to gain experience on how to bid and how to play a hand. People do very poorly when picking up the game, but if you keep playing, your skills increase until you can begin to hold your own.