Here's how you can know for certain that you'll be in Heaven when you die. This verse says quite clearly that you can know for certain that you have eternal life.
Eternal life is salvation.
Salvation is spending our eternity with Christ in Heaven after we die. There is nothing that we can ever do to earn our salvation. There is no task great enough or no work good enough to reserve us a place in Heaven. Salvation is a gift from God. It is a free gift that God makes available to us by His grace.