I am a Muslimah in my middle teens. I am trying to be a good Muslimah, but I have a problem. I waste too much time and do not know what to do. Please suggest something helpful that will keep me busy and not lower my faith and goodness.

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

May Allah increase your reward your striving to follow Islam and benefit from time. We ask Him grant to you and us success, guidance and steadfastness on Islam.
As for your question concerning wasting time, know that time is the dearest from among a human being's possessions. So, one has to be more keen to value time than wealth.
Here are some suggestions that, Insha Allah, will help you benefit from time:
1) Spare no effort to perform obligations, to do more supererogatory deeds such as prayers, fast, charity and to avoid prohibited, disliked and suspicious matters.
2) Dedicate a daily fixed period of time for reading the Qur'an as well as memorizing some portions of it.
3) Consecrate a period of time for reading useful books and listening to beneficial recorded materials.
4) Arrange to meet with righteous females, and attend lessons and gatherings of knowledge.
5) Devote a daily period of time for studying your school assignments and homework.
Allah knows best.