You can be 100% sure about Heaven,
it's simple as A, B, C

Admit that you have sinned against God,

* For all have sinned, and come short of the glory of God. Romans 3:23

* As it is written, there is none righteous, no, not one; There is none that understandeth, there is none that seeketh after God.
 
* They are all gone out of the way, they are together become unprofitable: there is none that doeth good, no, not one.  Romans 3:10-12

         
   and that there is a severe penalty for your sins.

* For the wages of sin is death...Romans 6:23

*...your iniquities have separated...you and your God, and your sins have hid His face from you, that He will not hear. Isaiah 59:2

* But the fearful, and unbelieving, and the abominable, and murderers, and whoremongers, and sorcerers, and idolators, and all liars, shall have their part in the lake which burneth with fire and brimstone: which is the second death. Revelation 21:7

Believe that Christ paid the penalty for your sins!

* Christ died for our sins according to the scriptures; and that He was buried, and that He rose again the third day according to the scriptures...1Cor 15: 1-8

* 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. John 3:16

* But God commendeth His love for us, in that, while we were yet sinners, Christ died for us. Romans 5:8

* Neither by the blood of goats and calves, but by His own blood He entered in once into the holy place, having obtained eternal redemption for us. Hebrews 9:12
Confess to God that you have sinned against Him, repent (turn away from) your sins, and ask Him to forgive you.

* 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.

* For with the heart man believeth unto righteousness; and with the mouth confession is made unto salvation.

* For whosoever shall call upon the name of the Lord shall be saved. Romans 10: 9,10,13

* If we confess our sins, He is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness. 1 John 1:9

* "He will never leave you, nor forsake you." Heb 13:5