Don't let your fear of failure, or anything else, keep you from sharing your faith in Christ with those who need to know Him. How else will they learn about His love if someone doesn't tell them?
No, you may not be able to answer all their questions (although you could help them find answers if their questions are sincere; your local Christian bookstore can suggest books by scholars who have dealt with them). But you can tell what Jesus Christ means to you, and how He has changed your life. They might argue with the answer to some deep philosophical question -- but they can't argue with the testimony of a changed life!
The most important thing you can do for your friends, however, is to pray for them. You can't open someone's heart to the truth of the Gospel -- but God can, by His Spirit. Pray also that God will give you opportunities to share Christ with them. Paul wasn't eloquent, but God still used him because he depended on the Holy Spirit to guide him (see 1 Corinthians 2:1-5).
Ask God also to help you be a witness by the way you live. Do others see Christ in you -- in your attitudes, your motives, your language, your character? Remember Jesus' words: "You are the light of the world.... In the same way, let your light shine before men, that they may see your good deeds and praise your Father in heaven" (Matthew 5:14,16).