The first pillar of Islam is to pronounce the Shahada:

Ashadu an la ilaha ila Allah, wa ashadu anna Muhamad rasulullah

The belief and testimony that there is no God but Allah and that Muhammad is His messenger. The definition of the shahada is express with the tongue and believe with your heart. But what does it mean to believe in Allah? Believe in Allah entails accepting His existence, to know His attributes and believe in them. It is obligatory for every Muslim to believe in the existence of Allah, that He is before the beginning of time and has continuity beyond the end of time. Belief that He is unique, there is only one, it is self-sufficient, independent, He is not in need of absolutely anything, that He is different from all that He has created, that He is the Living, the Mighty, the Wise, the one with will on all things, He has the hearing, sight and speech. These are some of the attributes of Allah, in which believe is obligatory, and therefore is also obligatory to deny all opposed to these attributes. To know and believe in these attributes, we are confirming the first part of the Shahada, the reference that Allah is the only God: Ashhadu an-la ilaha il-la Allah, I testify that there is no God but Allah.

The second part is to bear witness that the Prophet Muhammad is the last of the messengers, the seal and culmination of all the prophets, sent by the Creator both men and jinn, He ratified the messengers before him, and upon him Allah sent down the Qur’an as a guide for men: wa Ashhadu an-na Muhammadan Rasulul-lah, and I bear witness that Muhammad is the Messenger of Allah.

Accept Islam is to say with sincere and self-belief the shahada before Muslims witnesses and from that moment becomes obligatory the four other worship practices or pillars of Islam: The Salat, Zakat, fasting in Ramadan and pilgrimage.