Hi Ilham, I'm really pleased to give you more info. To make it easier I'm building a blog with pictures, prices and info about the area.
Please check this website so you can get a better idea http://sardiniaccommodation.blogspot.com/
Referring to the car, I will suggest to hire one if you like to go around a bit more freely. North sardinia has amazing panoramic roads and some great beaches have access just by car (not public transports). Anyway, posada (the village I'm renting the flats in and where I go myself to spend the summer) is well connected with public transports as there are coaches running every hour from the airport of olbia (have a look to the section in the blog: getting around) and once you are in posada you can walk or cycle to the beach.
Once you have a look at the prices and pictures, write me a mail so I can send you my phone number
[email protected]
Have a nice day
Marila