Short answer
To become a Muslim, a person says the shahada with sincere belief: 'I bear witness that there is no god but God, and I bear witness that Muhammad is the messenger of God.' No ceremony, priest or special place is needed, and it can be said alone.
The fuller answer
The shahada means believing in one God alone, and accepting Muhammad ﷺ as His final messenger and what he brought. Many people say it in front of witnesses at a mosque or Islamic centre, which is encouraged, to receive support, but it is not required. A new Muslim is not asked to be perfect or to know everything.
After this, a person begins to learn the five daily prayers, fasting in Ramadan and the basics of Islam, step by step. The Prophet ﷺ said that Islam wipes out what came before it, so all of a person's past sins are forgiven. Many new Muslims also take a bath (ghusl) and some choose a new name, although a name change is not required.
This answer follows the mainstream Sunni understanding and is general learning, not a fatwa. Where Muslims differ, the answer says so. Have a question of your own? Write to us.