Barcelona is my favorite spot for summer vacations. Any time I have a chance I go there for my holidays. This is why I've visited most of the beaches there. I guess this is your first time there and this is why you are asking what you should do. Well, as a beginning you can try staying at a hotel. Usually when I'm leading discussions on accommodation I'm always saying to the person that it would be better to rent an apartment but since Barcelona is worldwide known and thousands of tourists are visiting it every year the prices of apartments are extremely high. Barcelona apartments can be compared to apartment Valencia and Apartments Lisbon. They are really expensive and this is why I would recommend you to book some hotel in advance. If you manage to do so you might even take advantage of a discount.
So the beaches are numerous and they can be divided on what you are currently seeking - party or quiet place. For more private and quiet place I would recommend you to go to Vilanova I geltru. The place has some really beautiful beaches and they are not that popular. I'm sure that you will have a great time there.
Sitges is also offering some nice beaches but the party there is rocking. There are lots of young people, bars and places worth visiting. If you are a party type of person this is definitely the right place for you. It is located on hour, hour and a half from Barcelona so you can go there even by public transport. You don't need to own a vehicle. As a whole there are plenty of beaches over there and even if you don't have an idea where to go you will enjoy all of them because the place is like heaven.
But anyway the closest beaches to Barcelona are Barcelonata which is known as the urban beaches. It is a nice beach with lots of bars and cafeterias.
Second one is Icaria beach - it is accessible even by metro and this is why many people prefer going there. It is quite fun and easily accessible. Number 3 is Poblenou metro station. You can reach it by walking 15 minutes from Poblenou metro station. And number 4 is Sitges which I mentioned earlier.
Personally my favorite beach is Marbella beach. It is not a nudist one - of course there is a small area for those kinds of people but as a whole the beach is just great and you can find it pretty attractive because of the lots of fun places you can visit there. I don't know how much time you intend to spend in Barcelona but my advice to you is to stay there as long as you can because the place is just marvelous. If you don't believe me you will be convinced in my words the minute you step there. I hope you have a great time and I'm sure that this summer will be one of the best in your life.