How Does One Become a Muslim?
In order to become a Muslim, there are no specific religious rituals or customs that you need to perform, either in specific areas or in the company of others. This is because, in Islam, a person has a direct relationship with his Lord without any intermediaries.
By simply believing and declaring that there is only one God and that Muhammad is His servant and Messenger, one thereby becomes Muslim. Upon making this statement sincerely, every sin in someone's life up to that point is forgiven.
God says in the Holy Qur’an:
Except for those who repent, believe and do righteous work. For them Allah will replace their evil deeds with good. And ever is Allah Forgiving and Merciful. 
(25:70)
You begin life again as a Muslim, one who submits to the will of God.
As for non-Muslims who have accepted Islam, they will be given their reward in double, due to their belief in their messengers as well as their belief in Muhammad 
.
Allah says:
Those to whom We gave the Scripture before it – they are believers in it. And when it is recited to them, they say, ‘We have believed in it; indeed, it is the truth from our Lord. Indeed we were, [even] before it, Muslims [submitting to Allah].’ Those will be given their reward twice for what they patiently endured and [because] they avert evil through good, and from what We have provided them they spend. (28:52-54)
In addition to this, Allah effaces all the sins they did before accepting Islam. The Prophet said: “Islam effaces all [the sins one did] before it.” (Muslim).