You Are a Muslim When You …
Believe in your heart that there is only ONE God (named Allah in Arabic).
Believe in ALL of Allah's prophets from Adam to Muhammad, peace be upon them all making no difference.
Understand that all prophets came with the same message, which is to believe in God and to do well in this world to be rewarded possibly in this life and certainly in the hereafter.
Remember that a Muslim is someone who the people are safe from his hand and his tongue (Prophet Muhammad's saying).
When you believe in that, you have to make The DECLARATION (Al-Shahada) by tongue and by heart.
I Bear Witness that there is No god, but Allah
Ash-hadu Anna La-Ilaha Ella-llah
Islam is not only beliefs and words. Actions must support the words.
Pillars of Islam are: Testifying Al-Shahadah, Performing daily Prayers, Paying Zakat (Mandatory Charity), Fasting during Ramadan, Performing Pilgrimage to Mecca if one is financially and physically able.
Performing prayers is the first duty of a Muslim. The prayer is mainly recitations of verses from the Quran "The Holy Book". The recitation is in Arabic in order to assure that all Muslims in the world perform the prayers the same way without giving room to translation distortions.