Firstly, it is always best to get someone who is a native speaker to correct your work. In this instance, it is fine to leave the tense as present because of the context, but adding 'on' makes this clearer. I know how difficult it is to translate and you did a good job.  

Summer holidays!

I tried to translate a text from my english grammar book!
It will be full of errors, I know that I have to study english prepositions a lot more/spend more time studying english prepositions!!! =P

Every summer I stay in at a camping ground located in Numana for a month. We often leave on sunday afternoon at about 2 o'clock and we arrive there before dinner time. It takes almost four hours to reach Numana from my home. That day (Or on the day of our departure)  I wake up early and I help my dad to put all the camping gear into the car. After a light lunch, my father always says: "We'll move in five minutes!". In a minute (or an instant) we are ready and we can go into the car: dad is at the wheel, mum is next to him, my sister and I sit behind, among plastic bags, hats and towels. When we arrive in Numana, my sister and I turn off(I think you mean get out of the car) the car and run to the beach to say hello to our friends. There is always someone under a beach umbrella or (hanging?)around the bar. Then I help dad to put up the tent, while my mother and my sister get the luggage out of the car. When everything is in order we have dinner in a little restaurant not far from the camping ground where we stay. Obviously we go there on foot. After dinner we go to sleep immediately. Our holiday starts the next day. In the morning, after a big breakfast, I go around the campsite to call all my friends. Then we go to the beach together and we stay there till lunch. We usually play volleyball, we swim or we listen to the music. In the afternoon I meet my friends in the bar at the campsite and we go back to the beach. Sometimes we take our rented bicycles into Sirolo, a beautiful town near Numana. Otherwise, if George is with us, we can go in his car. On the rainy days the place where we meet is the recreation room in the city centre. In the evening, after dinner, we usually walk around the city or we climb the hill to see the landscape. Then we go to the disco. Otherwise, if there is a birthday or an anniversary we have a barbecue on the beach and we stay there until the sun rises. Summer holidays are really funny. The only bad thing is that time runs out and it is already time to come back home.