What Do You Say To Declare Your Conversion To Islam?

To achieve this, it is sufficient to say (in Arabic, if possible):
’Ash-hadu ’an la ilaha ’illaal-lah!
Wa ‘ash-hadu ‘an-na Muhammadan Abduhu wa rasuluhu
The meaning of this is as follows:

I bear witness that there is no (true) god except Allah.
And I bear witness that Muhammad (P.B.U.H) is Allah’s Messenger.
Also you can say:

I (firmly) believe in Allah, His Angels, His scriptures, His Messengers, the Last Day, and in the ability of Allah to will the existence of good or bad.
I promise Allah not to associate with Him (in worship) anyone or anything (whatsoever), and,
I will steadfastly perform Salat (the Five Prayers), and
I will give out Zakat (special Islamic system of charity), and
I will never steal, and
Never commit adultery (or fornication), and
Never illegally kill a person, and
Never disobey Allah.
Praise be to Allah Who has guided me to belief !
Note: The person in charge, in front of whom a new Muslim has announced his conversion to Islam, prays Allah for him with such prayers: Allah may forgive me and you! And Allah may accept (submission) from me and you! O Allah! Accept him with those on whom You did bestow your Grace, of Prophets, the Sincere (lovers of Truth), the Witnesses, and the righteous! What a beautiful Fellowship!