Use the search function and type in the question or topic and check out the results. Additionally, you can go to one of the big Christian answers sites listed below and choose from hundreds of common questions. Believe me, no matter what a skeptic comes at you with, there are most likely ready answers from these resources that you can grab.

Oh, one other thing, if it's apparent that the skeptic is just messing with you, don't waste a lot of time trying to answer every concocted objection. Remember the scripture that exhorts you not to cast your pearls before the swine. If you perceive the person might be a genuine seeker then prayerfully and with much love and humility try to help them find the answers to their questions. On the other hand, if they are a commited detractor but they are trying to influence others against the Christian faith, then answer the detractor as you would an honest seeker in the presence of those being influenced. Use your discretion and may God richly bless your efforts in Him!