After having decided to revert to Islam, one must sincerely utter a testimony called shahadah. The Shahadah consists of two parts. One is to testify that there is no deity (god) worthy of worship except Allah. The second part is to testify that Muhammad (pbuh) is the final messenger of Allah.
The shahadah is not necessarily to be said in Arabic, though preferable. One may say the shahadah in front of two witnesses, but one should not delay the testimony. It is preferred that after taking shahadah, one takes a bath.
The shahadah reads as follows:
“ASH-HADU ALLA ILAHA ILLALLAH WA ASH-HADU ANNA MUHAMMADAN RASUL-ALLAH".
The Translation: "I bear witness that there is no deity (none truly to be worshipped) but Allah, and I bear witness that Muhammad is the messenger of Allah"
After taking the shahadah, one must start the way towards God by following the five pillars of Islam.