Learning another language is not only limited to studying from books and taking classes. Immersing yourself in an English-speaking environment, whether you are in the US or at home in Indonesia, will help to build your vocabulary and improve your listening skills.
By listening to music in English, watching television/movies in English, becoming friends with native English speakers in your local community and effectively using the websites listed below, you will be able to continue to improve your overall English ability and prepare for standardized exams.
Admission Exam Preparation
TOEFL Preparation Materials
GRE Preparation Materials
GMAT Preparation Materials