We Catholics have traditionally lagged far behind our separated brethren in Protestant communities in the area of sharing our faith. While your average American Evangelical is raised from an early age to share their faith with anyone who would listen (and many who wouldn't), it wouldn't occur to the average Catholic to talk about what their faith meant to them personally or share the particulars of it with the aim of leading someone to Christ.
Fortunately, thanks to the New Evangelization of Blessed John Paul II and the Holy Father, more and more Catholics are learning the importance of sharing their faith with others. What stops them is not knowing how to begin. It does not advance the cause of Christ to tell someone about Our Lady of Fatima who may have never heard of Jesus in the first place.
As a former evangelical, one of the things I learned early on was that most of the time it wasn't what you said, but how you said it. Even if the presentation is not polished or theologically sophisticated, people are more likely to listen if they are met approached kindness, sincerity, and understanding. There is an old saying, "people don't care how much you know until they know how much you care."
Over at National Catholic Register Online, Matthew Warner of Fallible Blogma has a great posting covering 10 Dos and Don'ts of Online Evangelization. While he is talking specifically about sharing your faith on Facebook, Twitter, blogs, chatrooms and the like, they are still good tips for face-to-face evangelization.