1 Peter 3:15 says, "always be prepared to give an answer to everyone who asks you to give the reason for the hope that you have. But do this with gentleness and respect." (NIV)
The Center for Public Christianity, out of Australia, has an amazing website full of resources targeting the various questions unbelievers inevitably ask. Check out the site, peruse the various topics listed below, and bookmark it for future use. Includes a wide range of audio, video, and print materials.