Becoming Muslim is one sincere declaration: no ceremony, no paperwork, no prerequisites. Here it is in Arabic, in letters you can say today, and in plain meaning.
أَشْهَدُ أَنْ لَا إِلَٰهَ إِلَّا اللَّهُ، وَأَشْهَدُ أَنَّ مُحَمَّدًا رَسُولُ اللَّهِ
Ash-hadu an lā ilāha illa-llāh, wa ash-hadu anna Muḥammadan rasūlu-llāh.
“I bear witness that there is no god but Allah, and I bear witness that Muhammad is the Messenger of Allah.”
If you believe it and said it, you're in. Welcome. That's the whole threshold. The rest of the path is gradual, but you don't walk it alone: start here, and consider telling a nearby masjid you're new. It reliably turns into tea and phone numbers.
No. The shahada said sincerely, even alone in your room, makes you a Muslim. Saying it at a mosque with witnesses is common and beautiful because the community welcomes you on the spot, but it is a celebration, not a requirement.
Say it in Arabic as best you can (the transliteration above is enough), and know what it means in your own language, the meaning believed in the heart is what counts. Pronunciation polish can come later.
Your past sins are wiped away and you begin with a clean record. From there, everything is gradual: learn wudu and the prayer step by step, and find a community, no one expects completeness on day one.
Its two halves carry the whole faith: worship belongs to Allah alone, and Muhammad ﷺ is His messenger, so the Qur'an he delivered and the way he practiced it become your guide. Everything else in Islam unfolds from those two convictions.