Any human being can become a Muslim, no matter their previous religion, age, nationality or ethnic background. To become a Muslim, you simply say the following words: “Ash hadu alla ilaha illa Allah, wa ash hadu anna Mohammadan rasulAllah”. This testimony of faith means: “I testify that there is no god worthy of being worshipped other than Allah and that Mohammad is His Messenger”. You must say it and believe in it.

Once you say this sentence, you automatically become Muslim. You do not need to ask about a time or an hour, by night or by day, to turn to your Lord and begin your new life. Anytime is the time for that.

You do not need anyone’s permission, or a priest to baptize you, or an intermediary to mediate for you, or anyone to guide you to Him, for He, may He be glorified, guides you Himself. So turn to Him, for He is near; He is closer to you than you think or can imagine:

And when My servants ask you, [O Muhammad], concerning Me - indeed I am near. I respond to the invocation of the supplicant when he calls upon Me. So let them respond to Me [by obedience] and believe in Me that they may be [rightly] guided.

Quran - 2:186

(interpretation of the meaning)

Allah, may He be exalted, says:

So whoever Allah wants to guide - He expands his heart to [contain] Islam; and whoever He wants to misguide - He makes his heart tight and constricted as though he were climbing into the sky...

Quran - 6:125

(interpretation of the meaning)

Source: islamqa.info · islamqa.info