The shahāda
Belief in two truths, said with the tongue and held in the heart. The doorway every Muslim walks through, said again at the close of every prayer, and ideally said at the close of every life.
Ash-hadu an lā ilāha illallāh, wa ash-hadu anna Muḥammadan rasūlullāh.
I bear witness that there is no god but Allah, and I bear witness that Muhammad is the Messenger of Allah.
From the Sunnah
The first pillar
The shahāda is the first of the five pillars of Islam. Sincere belief in it makes someone a Muslim.
Bukhārī 8, Muslim 16
The key to paradise
The Prophet ﷺ said: 'Whoever's last words are lā ilāha illallāh will enter paradise.'
Abū Dāwūd 3116, ṣaḥīḥ
The heaviest deed on the scale
On the Day of Judgement a man whose record fills 99 ledgers of sins will have a single card placed on the other side bearing the shahāda — and the card will outweigh them all.
Tirmidhī 2639, ṣaḥīḥ
Saying these words sincerely, with belief in their meaning, is enough. There is no required ceremony. Many take it in the presence of two Muslim witnesses so the community can welcome them, but that is custom, not a condition.
If you've taken the shahāda or are about to: welcome. The first thing to learn next is wuḍū and the five daily prayers.