You can know for sure.  There are only four things you need to do.  

First, realize where you are:  

Romans 3:23 

for all have sinned and fall short of the glory of God,

 Romans 6:23 

For the wages of sin is death, but the gift of God is eternal life in Christ Jesus our Lord.

 Second, acknowledge that you cannot save yourself. 

Ephesians 2:8-9 

For it is by grace you have been saved, through faith--and this not from yourselves, it is the gift of God--not by works, so that no one can boast. 

Third, see what God has done for you.  

Romans 5:8 

But God demonstrates his own love for us in this: While we were still sinners, Christ died for us. 

John 3:16 

"For God so loved the world that he gave his one and only Son, that whoever believes in him shall not perish but have eternal life. 

Fourth, receive Jesus as your Savior.

 Romans 10:9 

That if you confess with your mouth, "Jesus is Lord," and believe in your heart that God raised him from the dead, you will be saved.

 Romans 10:13 for,

 "Everyone who calls on the name of the Lord will be saved." 

Pray this prayer;  

  "Dear Jesus, I know I am a sinner.  I know that without you I will not go to heaven, but will surely go to hell.  I am sorry for my sins, and ask you to forgive me- of sins I have done knowingly, and also those I have done in ignorance.  I give my life to you and ask you to be the Lord of my life.  I give you my heart and ask you to live in me.  Save me and help me live for you,  I desire to follow you all the rest of my life.  I love you, Lord.  In Jesus name, I pray, Amen.  

     Baptism 

It is a symbol of our new life in Him.

 Romans 6:4 We were therefore buried with him through baptism into death in order that, just as Christ was raised from the dead through the glory of the Father, we too may live a new life. 

It is an example of the believers and their act of obedience to Him.  

Acts 2:41 

Those who accepted his message were baptized, and about three thousand were added to their number that day. 

It is commanded by Jesus.  

Matt 28:19-20 

Therefore go and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, and teaching them to obey everything I have commanded you. And surely I am with you always, to the very end of the age."

We have an assurance of our relationship with Him and our hope of heaven.

Romans 8: 35-39 

Who shall separate us from the love of Christ? Shall trouble or hardship or persecution or famine or nakedness or danger or sword? As it is written: "For your sake we face death all day long; we are considered as sheep to be slaughtered." No, in all these things we are more than conquerors through him who loved us. For I am convinced that neither death nor life, neither angels nor demons, neither the present nor the future, nor any powers, neither height nor depth, nor anything else in all creation, will be able to separate us from the love of God that is in Christ Jesus our Lord. John 10:28-29 So Jesus said, "When you have lifted up the Son of Man, then you will know that I am the one I claim to be and that I do nothing on my own but speak just what the Father has taught me. The one who sent me is with me; he has not left me alone, for I always do what pleases him."  

 John 6:37-39 

All that the Father gives me will come to me, and whoever comes to me I will never drive away. For I have come down from heaven not to do my will but to do the will of him who sent me. And this is the will of him who sent me, that I shall lose none of all that he has given me, but raise them up at the last day.