Are you sure that you will go to Heaven when you die? There is not a set prayer in the Bible to help you become part of God’s Kingdom but you will not doubt again that you are a child of God when you pray according to the Bible verse given.

As many as received Him, (Jesus) to them He gave the right to become children of God, to those who believe in His name.
John 1:12

Lord Jesus, I believe You are the Son of God.
Thank You for dying on the Cross for my sins.
Please forgive me.

I receive You now Jesus, as my Saviour and Lord.
Make me a child of God.
Amen