as for today i am 18 years old and i am an adult. but i dont realy feel like i matured so much to hava responsibilities towards me and other peole.also because i lost a year coming to a new country.i am the eldest in my class .so i dont how should i built my relashionship with my classmates and more particulary with the girls because there are not in my age and this may cost my studies and future in a foreingh country.how should i act.

;;;;;;;;;;please give some good advices if its posible.

Act like yourself. Teens love exchange students.

And what society plays as 'mature' such as mature content ISN'T actually maturity. It's actually content that isn't for anyone. It's much more mature to stay away from M rated or R rated things.
Though, I'm not saying to delete them from your life, I'm just suggesting good judgement, now that the world regards you as an adult.

18 =/= Mature

Mature = Mature

It is mandatory to 'Grow Old'
it's optional to 'Grow Up'

People are expected to be mature when they are 15, 18 is when people start to treat you as if you were.

As for advice, the best i can give you is be your self, if you don't like something to the point you would ask help, you should already know you wan't to change it.