We need to be able to explain what we believe and why we believe it. It's not hard to do, but it does require some effort on our part. The best place to get this is from books, but that's not the only place. There are radio programs and podcasts that cover this as well as online classes. I highly recommend BiblicalTraining.org's classes on the Bible, theology, and apologetics — all free.
But don't run away from reading. If the average reader can spend just 30 minutes a day reading, he can go through 5 or so good books a year. And there are books on tape/mp3 (visit ChristianAudio.com). Some Kindles will read to you, too. (I have my Kindle plugged into the aux port in my car stereo, and it reads to me every day as I commute to and from work.)
In short, there are a lot of ways you can equip yourself if you’re willing to put the time and effort into it. After you start learning, you need to practice. Basically, start up discussions with your friends — get used to using the information, and try to learn the arguments that can come up against them. You also might want to read a little anti-Christian literature. This is an area where it is important to test yourself against the other side or you won’t grow. So at first find Christian friends to play devil’s advocate, and then find people who are actually on the devil’s side. It’s a painful growth process, but you’ll be glad you did.
Preparation is the key here. You have to prepare before you’re in trouble. If you wait until you need this, it’s too late.
Last, after you’ve prepared, and after you’ve practiced, repeat! If you don’t use it, you lose it. Think about all that math you used to know. It's gone now due to lack of use, right? If you don’t keep refreshing yourself, you’ll start to forget.
So what should you read? Theology and apologetics. What does that mean?
Theology focuses on drawing what we believe out of the Bible. Systematic Theology tries to gather up everything the Bible says on certain topics. This is mostly what I'm talking about (not that other types of theological study don't have value, too). There are good books on certain theological topics (eg, the nature of God or the deity of Christ) — both large and small. There are a number of good single-volume systematic theologies. There are also multi-volume sets. You can go as shallow or as deep as you want. You could just read one quality systematic theology and call it done, or you could read a few and see what differences there are and work out what you personally believe. I recommend reading at least one single-volume systematic theology.
5-Minute Theologian is the shortest book I've found that can be called a systematic theology. It's the bare minimum, but it's something. Read one short "5 minute" chapter a day and you'll have a fair survey of systematic theology in just over a month.
But if you can go a little heavier, there are good single-volume texts by Ryrie, Grudem, Erickson, and Geisler. (A word of warning, most systematic theologies are written by theologians of a Calvinist bent; they're all written from some theological system or another. Don't let it bother you if they don't teach everything you believe.) Some are written for a popular audience, and some are aimed at theology majors or even graduate students; check descriptions and reviews so you aren't surprised, but while the popular may give you all you need, you might find you enjoy the deeper studies.
Remember that theology is just what we believe. Some people can make it dry, but studying the nature of God, the person and work of Christ, and the ministry of the Holy Spirit can be powerfully moving. You will probably find yourself drawn to worship as you read about the working of God. It's not for nothing that Paul so frequently slipped into doxology as he explained God's nature and plans.
As for apologetics, you can break it up into different topics. What I call historical apologetics refers to questions about whether or not the Bible is reliable or whether Christ really rose – things like that. A great introduction to that topic is Lee Strobel’s The Case for Christ. Besides being a good book, its best feature is that at the end of each chapter on the different topics it gives you a good list of books to read if you want to go deeper. Josh McDowell’s More Than a Carpenter is a popular and shorter book, though it’s narrower in focus.
Scientific apologetics gets into questions regarding the origin of the universe and the origin of life. Strobel has another of his survey/introduction books called The Case for a Creator that you might want to check out. Two books I really got a lot out of are Hugh Ross’ The Creator and the Cosmos and The Fingerprint of God.
(I should probably step off on a tangent here. Ross is what is called an old-earth creationist – he believes God made everything billions of years ago. Many evangelical Christians are what is called a young-earth creationist – they believe that God made everything a few thousand years ago. I don’t care which you are as long as you believe God created everything. But if you are a young earther, and you come across a typical non-Christian who believes the earth is 4 billion years old, your choices are to spend a lot of time trying to convince them that everything he believes about the universe is wrong or you can use what he believes to show that God must have created the universe. The second is a lot more likely to succeed. Believe whatever you think is true, but meet your atheist friend where he is.)
Philosophical apologetics is really just answering those more generalized questions – does God really exist, how can a good God allow evil, things like that. C.S. Lewis’ Mere Christianity is considered one of the classics of the field; it also gets into some theological stuff. The oh so prolific Lee Strobel has another book called The Case for Faith that deals with these kinds of questions too. William Lane Craig's On Guard covers this well with a bit on the reality of the resurrection as well; it's my new favorite.
5 Minute Apologist (written by the same author as 5 Minute Theologian) has a book with those same brief chapters that covers the gamut on apologetics in obviously less detail, but if that's all you can do, or if you need a primer to get started, do what you have to do.
These are just my suggestions. There are lots of great books out there, and if you’ve got some, or if someone’s recommended some others, that’s fine. This is a resource for those who don’t have any idea where to start.
This is all general stuff. If you need to answer questions regarding a particular religion or cult or issues impacting our society right now – like the stem cell debate – you may have to do more specific reading. But if you’ve never read a single book on the Jehovah’s Witnesses, yet you’re very familiar with Christian theology, you will be equipped to deal, at least on a basic level, with JW questions that arise. We can’t be specialists in everything, but we can be broad generalists, and that will help you know where to go when you need more specific information.
What matters is to prepare. Decide you're going to do it, buy a book or download some mp3's, and set a time to go through the material. The other side's information is everywhere, ready to assault hearts and minds. You are the defense. Arm yourself.