How old do you think you should be to start witnessing?
You are a witness your entire life. What you do and how you act is a testimony for good or bad for Christianity.
As far as specifically talking with someone about Christ, I'd say seek to be filled with the Holy Spirit and then follow His leading. If He directs you to witness to someone, then you should do it. It doesn't have to be intense.
I would suspect that you would be witnessing to people who are near you in age, so the questions you are likely to be asked are ones you can probably answer. If you can't answer a particular question, you can look up helps online, in a Bible concordance, or talk to an older Christian you know to help you with the answers.
Getting involved with a Sunday School program is an excellent way to learn. The Small Group setting can be another good way, but all too often the kids play while the adults discuss. This won't help you learn like a good Sunday School class.
Sadly, many Sunday School classes are catering more and more to the visual aid watch a DVD like a lump on a rock approach that isn't very instructive because they don't want a teacher to say the wrong thing or a kid to actually have to think. Just because the newer generation is more on-line and connected doesn't mean they actually learn anything worthwhile that way.
The reality is that we are all called to spread the Gospel's Message of salvation through the blood of Christ. Your life will each intersect with a unique group of individuals that perhaps no other person will ever reach.
Follow the Holy Spirit's prompting and you'll do fine.