which area of barcelona is the best part to stay (getting hotel) for young couples?

having good night life,resturant,bars.

Any place right along the Ramblas will probably do for you. If you need quiet to sleep, look a block or two off the Ramblas, you can walk to your party places, then escape when it's time to go to bed. Closer to the beach is probably better. More partying that way.