I went to Duke Divinity School. So I always have to remind myself that not everyone, as a result of their seminary training, was introduced to Karl Barth the way I was. So it's important I think, considering the size of his magnum opus -- Church Dogmatics -- to find ways working pastors might become familiar with Barth, even if just for learning his biblical and theological accuracy.
So, where to begin?
If you're a preacher and would like easier access to Barth's theology, here are my recommendations.
1. My first semester at Duke, back in the day, we were required an introductory seminar in "The Church's Ministry". Part of our reading for that semester was Karl Barth's Evangelical Theology, a compilation of lectures delivered in 1962. Barth, of course, was Swiss. So he spoke German. But his English was meticulous. Thus, this is an exciting and exquisite introduction to Barth's mature thinking. Above all, it highlights the joy of being a theologian. I can't recommend this book any more highly than that.
2. I recommend Barth's commentary on that most ecumenical of Protestant catechisms, The Heidelberg Catechism, titled The Heidelberg Catechism for Today. You can also pick up a later release of this material titled Learning Jesus Christ Through the Heidelberg Catechism which contains very similar content. As a pastor, it's one of my joys to offer Catechism classes every year when I'm in the parish. And I've taught from the Heidelberg Catechism in my college theology classes. Because the Catechism takes great pains to demonstrate how faith is our very living, not just our thinking. And Barth's commentary helps us see how Jesus Christ, through the lens of the Catechism, is our very living.
3. Then there's Barth's Faith of the Church: A Commentary on the Apostles' Creed According to Calvin's Catechism is an excellent interpretive version of the Apostle's Creed, through the lens of the Swiss Reformer John Calvin's catechism based upon it. Confused yet? This is Barth's take on Calvin's take on the Apostle's Creed, one of the foundational creeds of the Christian faith. I would highly recommend this volume to anyone, but especially to those who are interested in Barth studies and don't really know where to start. Due to its non-academic nature, this book provides a great introduction to a very dense thinker. It's also quite devotional, which is a plus, and can be read profitably as a pastor's own devotional material. (If you want less Calvin and more creed, there's Barth's Dogmatics in Outline. I've also used this little gem in my theology classes. It's highly accessible and well worth the investment.)
4. Pastors preach. So I can't overlook Barth'sHomiletics, a collections of lectures and seminars offered by Barth in which he reflects on how to prepare and preach sermons and how to interpret scripture. Consider it a master class with the great theologian. Perhaps I should list this one first, but I think it important to understand Barth's thought and theology beforehand.
5. Finally, the Church Dogmatics, though dense, still has help for pastors. I highly recommend taking a look at the Church Dogmatics Index Volume: With Aids for the Preacher which is the index of resources meant to aid and resource the preaching pastor (if you have access to the CD). The index includes a scripture and subject index to the Church Dogmatics, an incredibly helpful way into the material of the CD, as well as a section of "Preacher's Aids" which have commentary sections of sermon reflections by Barth on the preaching lectionary.
I still think, though, that the place every pastor should begin is with the Church Dogmatics themselves. I first read all 14 volumes in college, and found there more theological fodder than I've ever been able to out-grow. Barth's biblical exegesis is detailed and faithful. And there's enough material here to make the set an invaluable addition to any pastor's library. (The full set is now only about $130, from Hendrickson Publishers.) Even at the reasonable pace of 10 pages per day, you could read all of the volumes in about three years. I can't over-stress what a blessing this would be to your preaching and pastoral care!
At the very least, take a look at Barth's work on election and reconciliation (not to mention the doctrine of the Word of God) if you want places to start which are not to be missed.
There are, of course, plenty of secondary resources that make Barth accessible. But I'd begin with Karl himself.