The first pillar of Islam
أَشْهَدُ أَنْ لاَ إِلَهَ إِلاَّ اللَّهُ وَحْدَهُ لاَ شَرِيكَ لَهُ وَأَشْهَدُ أَنَّ مُحَمَّداً عَبْدُهُ وَرَسُولُهُ
Ashhadu an la ilaha illal-lahu wahdahu la shareeka lah, wa-ashhadu anna Muhammadan ʿabduhu warasooluh.
“I bear witness that none has the right to be worshipped except Allah, alone without partner, and I bear witness that Muhammad is His slave and Messenger.”
Hisn al-Muslim · Muslim 1/209
Its two parts
None has the right to be worshipped except Allah — affirming that Allah alone, without any partner, deserves worship.
Muhammad ﷺ is His servant and Messenger — accepting the Prophet ﷺ as Allah's final Messenger and following his guidance.
Accepting Islam
A person enters Islam by sincerely believing in the Shahadah and declaring it. There is no fee or formal ceremony — many choose to say it at a mosque, where the community can welcome and support them.
The wording above is the testimony the Prophet ﷺ taught to be said after wudu; the core declaration is the same.