First and foremost, just do your best to be a good example, and pray for guidance. When you are doing things that aren’t the “quick and easy path” and are really putting yourself out for others, or denying things that other indulge in, and you are truly not upset in doing so, it will invariably lead others to wonder why you are different than other people that they see. Sometimes providing an example does a lot more good than coming up with a good speech. Which is not to say that you shouldn’t talk about your faith. Talk about it, but don’t club him over the head mercilessly. But don’t back down when your faith comes up either.
Several of my friends aren’t specifically attatched to a church, and many people I know of on other forums aren’t Christians, and are, by thier definition, pagans. I don’t always bring up my faith or proselytize, but when matters of faith and morality come up, I try to do my best to represent the faith in as good a light as possible. Just remember, something little that you do now may not seem to have an affect, but you never know when that act of kindness or that particular explanation of your faith will “click” with the person you were witnessing to, and they will really feel a change in their lives.
Just don’t get frustrated if God takes his own time in revealing Himself to your aquaintance.