How to Know For Sure You Are Going to Heaven

For all have sinned, and come short of the glory of God; (Rom 3:23) 

This means that we have not just sinned against our fellow man, but against a holy and righteous God. Most people judge themselves by others they see and know. But God's standard is perfection. That is something that nobody can attain. If we have lied once, we are a liar in God's eyes. It doesn't matter if it is a “white” lie, a little lie, or a big lie. The point is that if we lie even once, we are a liar. It we steal something, irrespective of its value, we are a thief in God's eyes. If we even just look with lust, we are an adulterer at heart in God's eyes. The first step then is to realize that you are a sinner.

For the wages of sin is death; (Rom 6:23)

There is a penalty for sin and that penalty is eternity in hell. If that sounds severe, think about it in comparison to who you sinned against. A holy and infinite God. When we sin against God, we are sinning against an infinite Being. It only makes send that if you were a teenager for instance and hit your father, he would probably punish you. But if you then went out driving and were pulled over by a cop and hit him, the penalty would be higher. And if you were then brought before the judge for sentencing and hit him, you would face a more severe punishment. You see, as the authority of the offended rises, so does the punishment. If you sin against an I nfinite God, it will require an infinite punishment (i.e. hell). 

But God commendeth his love toward us, in that, while we were yet sinners, Christ died for us. (Rom 5:8)

God doesn't want anyone to go to hell. So He sent His Son to take our place on the cross and suffer and die for our sins. Jesus is an infinite being that can satisfy our sin debt. And He is fully man and was able to live a holy and sinless life for us. God can justly accept the sacrifice of Jesus Christ in place of punishing us since Jesus took our sins upon Him and died in our place. He then rose from the dead three days later defeating hell and death. God accepts us as we are when we come to Him by faith and ask Him to save us. BUT we must each personally ask God 

to save us and we must put our faith in what Jesus did on the cross. If we don't God cannot save us and MUST send us to hell. 

For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life. (Jn 3:16) 

For by grace are ye saved through faith; and that not of yourselves: [it is] the gift of God: ⁹ Not of works, lest any man should boast. (Eph 2:8-9)

We can't get to heaven by doing good works. God says in the book of Isaiah, ^“But we are all as an unclean [thing], and all our righteousnesses [are] as filthy rags; and we all do fade as a leaf; and our iniquities, like the wind, have taken us away.” (Is 64:6) In fact the Bible tells us that we must repent of “dead works.”

That if thou shalt confess with thy mouth the Lord Jesus, and shalt believe in thine heart that God hath raised him from the dead, thou shalt be saved. ¹⁰ For with the heart man believeth unto righteousness; and with the mouth confession is made unto salvation. (Rom 10:9-10)

According to this verse, we must confess that Jesus is Lord (i.e. He is Lord and we are not) and believe that He died and was raised from the dead. If we do this with the understanding that we are terrible sinners deserving hell, God will save us!