Can you be sure of where you'll spend eternity? Listen to these words from the Bible, in the book of 1 John:
And the testimony is this, that God has given us eternal life, and this life is in His Son. He who has the Son has the life; he who does not have the Son of God does not have the life. These things I have written to you who believe in the name of the Son of God, so that you may know that you have eternal life. (1 John 5:11-13, NASB)
Then the answer is clearly "Yes!" we can know where we will spend eternity. But let's keep going - the Bible has, actually, a great deal more to say on this question. Consider this passage as well:
For I am convinced that neither death, nor life, nor angels, nor principalities, nor things present, nor things to come, nor powers, nor height, nor depth, nor any other created thing, will be able to separate us from the love of God, which is in Christ Jesus our Lord. (Romans 8:38-39, NASB)
The first passage tells that eternal life is received by believing on the Son of God - Jesus Christ. If you're not sure you know exactly what that means, you can explore some of the other pages on this site. . .