How to know for sure you will go to heaven?

Admit that you are a sinner and that there is a penalty for sin

"For all have sinned, and come short of the glory of God" Romans 3:23 KJV
"As it is written, There is none righteous, no, not one" Romans 3:10 KJV
"For the wages of sin is death;" Romans 6:23 KJV

Believe that Jesus paid the price for your sin because you could never do enough to save yourself

”… but the gift of God is eternal life through Jesus Christ our Lord." Romans 5:8 KJV
"But God commendeth his love toward us, in that, while we were yet sinners, Christ died for us." Romans 5:8 KJV
"For by grace are ye saved through faith; and that not of yourselves: it is the gift of God: Not of works, lest any man should boast." Ephesians 2:8-9 KJV
"Not by works of righteousness which we have done, but according to his mercy he saved us …” Titus 3:5 KJV
"He that hath the Son hath life; and he that hath not the Son of God hath not life." 1 John 5:12 KJV

Call on Jesus to save you from sin’s penalty and know that you can never lose your salvation

"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 the scripture saith, Whosoever believeth on him shall not be ashamed." Romans 10:9-11 KJV
"For whosoever shall call upon the name of the Lord shall be saved." Romans 10:13 KJV
"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 KJV
"These things have I written unto you that believe on the name of the Son of God; that ye may know that ye have eternal life, and that ye may believe on the name of the Son of God." 1 John 5:13 KJV