There is really no short or easy answer to that question; however, there are two things that you can do to respond in an intelligent and responsible fashion.

1. Question the questioner.  Ask the person how/why they have come to that conclusion.  Truth is simply a coherent correspondence to reality; so, in what ways does the Bible NOT correspond to reality in a coherent fashion?  Challenge the person to cite either archaeological or historical evidence that contradicts any claim of scripture.  In general, persons who make these claims have not “done their homework” and are simply making a baseless claim that supports their unwillingness to “bow the knee”.
2. Study the appropriate sources.  The Case for (1) Christ, (2) Faith and (3) a Creator series, by Lee Strobel, are the most comprehensive and accessible books that deal with the nature of this question.  The manuscript evidence for the New Testament, the rational basis for faith in God and the scientific evidence for Intelligent Design by a personal Creator, are literally overwhelming in their scope and volume.

Most importantly, take to heart the words of 1 Peter 3:15.

…but sanctify Christ as Lord in your hearts, always {being} ready to make a defense to everyone who asks you to give an account for the hope that is in you, yet with gentleness and reverence;

The most important (and overlooked) aspect of this biblical admonition is that we should “sanctify Christ as Lord in our hearts”.  This is more than just being saved; it is a deep and serious commitment to walk in the ways and in the mind of Christ.  The second most overlooked aspect is the last phrase, “yet with gentleness and love”.  While the facts of the matter are a necessary part of preparing for discourse, without the proper relationship to Christ and the proper attitude, all apologetic efforts to persuade are ultimately doomed to fail because they are wrongly motivated and wrongly articulated.

Be certain that you are entirely comfortable with the reasons that you defend.  This is a necessary condition so that challenges to your faith are not seen as a threat.  When we feel threatened we most often respond in anger, and this is counterproductive to our witness.  We most often feel threatened by a challenge to our faith because we really don’t understand our own position as well as we should.  Apologetic resource materials such as the Strobel series mentioned above are just a start; a genuine and intimate relationship with Jesus Christ is the foundation of success in the great debates that pass our way.