i have been green card card holder for 6 years but i was outside usa, completing my studies had reenter permit ? currently i lived 7 months here

You must satisfy both the physical presence and continuous residence requiremebtsto be eligible for naturalization. The physical presence requires you to be present in the U.S. 30 out of the past 60 months. The continuous residence requirement requires you to have no absences of more than 6 months in the past five years.  You must have either filed income tax returns or be prepared to show no income tax return was required. You would not have to file an income tax return if you did not earn income. You can read more about naturalization at http://myattorneyusa.com/how-to-get-naturalized-in-the-united-states.