How can I grow in my understanding of sound doctrine?

Give heed to the preached Word. Listen attentively as your pastor expounds God’s Word each week. Discuss the sermon afterward with friends and family.
Study the Scriptures diligently. Aim for breadth and depth. That is, read through all of Scripture in order to see the big picture. But also, study smaller portions such as individual books, in the most detail you can manage. Read them over and over again. Meditate on them. Memorize them. Gaining a broad and deep grasp of Scripture is the most important way to grow in the knowledge of God.
Read good books. Ask your pastor for a list of the best theology books he knows and read them.
Discuss the Bible with mentors and friends. As you discuss the Bible’s teaching, apply it to your lives together.
Do evangelism. Explaining the gospel to non-Christians and answering some of their objections will strengthen your grasp on the gospel, sharpen your ability to communicate the gospel, and expose areas in your theology that you need to develop further.
Teach the Bible and theology to younger Christians. Teaching others, whether one-on-one, in a Bible study, in a Sunday School class, or even preaching, will compel you to study the Scriptures closely, understand them coherently, and communicate them clearly. All this will deepen your understanding of sound doctrine and enable you to use your knowledge to build up others.