Allow me to paint a scenario. Assume you are at school, or work, or chatting with your friends, or just talking to an acquaintance. Suddenly, your faith is placed in question when a teacher explains that the Big Bang resulted in the world and evolution took it from there, or that aliens brought life to earth. Your coworker adamantly proclaims that science disproves God's existence. A friend asks how you know that Jesus is the Son of God and that the Bible is trustworthy. The acquaintance laughs at the notion that Jesus rose from the dead. How would you respond?
So why should we learn to defend our faith? You'll find the answer to that question in a single verse, 1 Peter 3:15, "but in your hearts honor Christ the Lord as holy, always
being prepared to make a defense to anyone who asks you for a reason
for the hope that is in you; yet do it with gentleness and respect."
Put quite simply, we need to be able to stand up for and explain our Christian beliefs because the Bible commands us to do so. Similar to being "God's Billboard," we should know what we believe and why so that when asked, we are prepared to give a solid answer. By knowing scripture and studying, not only are we as Christians prepared to give a reason for our faith when put on the spot, but we are preparing our own minds to fend off any falsehood others will claim is fact. Instead of being placed in a situation where your faith is put on trial and you find yourself without an answer, be ready. Prepare now. Don't wait until a suicidal friend gives you five minutes to convince her that her life has true meaning before searching for the answers.
An example I believe demonstrates this well is that of a fireman, policeman, or soldier. They prepare so that when they are deployed, they will know what to do. They pour their time and energy into knowing everything possible about their field of work so that when the time comes, they're ready to put out the fire, handle dangerous situations, and fight the enemy. In order to defend our faith, Christians must learn how to do it, and know what their answers will be when the opportunity arises. Don't be caught unaware. There's abundant evidence for the truth of the Bible in astronomy, geology, biology, physics, archeology, and all aspects of science. If you aren't sure where to find these evidences, ask strong Christians who hold your own beliefs, locate Biblical institutions that interpret the Bible literally and offer clear evidences. Most of all, study your Bible to know truth for yourself—don't simply accept what you are taught, even from pastors and people you trust. Base your own beliefs on doctrines you can Biblically support.