It is simple. If you have already believed in One God, (Allah) the Almighty, He is the only ONE (no partner equal to Him) who Created everything in this universe and everything will return to Him, and you also believed in Prophet Muhammad (peace be upon him) is the last Messenger and Prophet of Allah, then you just need to proclaim Shahada (witness):
Reads (right to left in Arabic):
- The word of Testimony (Shahada)
اشْهَدُ انْ لّآ اِلهَ اِلَّا اللّهُ وَ اَشْهَدُ اَنَّ مُحَمَّدً اعَبْدُهوَرَسُولُه
Ash-hadu an-La illaha ill Allahu, wa ash-Hadu anna Muhammadan ‘abduhu wa Rasoolu
(I bear witness that there is no deity but Allah, and I bear witness that Muhammad is His servant and Messenger)
At the moment, you bear this witness, all your sins (no matter what it was) in the past will be totally erased in front of Allah. You are like a new born baby to start a new life. Congratulations and welcome to the new chapter of your life. Welcome to our sisterhood! May Allah give you protection and guidance to be successful in this life and hereafter.