Read. Read. Read. Smart people are readers. They engage in conversations not only with the people in their environment, but with good writers from other cultures and other times. You do not have to read classics. Read what you want. When I was a teenager, I read movie magazines, and TV tie in books. These are a step above comic books, but not a big step. The grammar in these books was very well-done, and my own grammar improved because I was looking at examples of good grammar every day. The writers may not have been the best, but they were professionals, and I was keeping professional company. I also had the reading habit.
I eventually graduated to slightly more sophisticated material. Today, I enjoy Stephen King, Dean Koontz, David Rosenfelt, and Janet Evanovich. Should I be reading Shakespeare and Mark Twain? Only if I feel like it, and sometimes I do. When I read for leisure, I pick the book and the author. Read one book a week. Keep a list of the books you read. Write a one page synopsis on each after you finish. No matter what books you read, your learning will get better.
As I write this I am just finishing Thomas Thompson's novel “Celebrity.” I have read this several times over the last thirty years. Thompson is a real pro. His characters are so well crafted that it is easy to believe they might be real people. His plot is full of twists and turns. He does everything a great writer should do while covering twenty-five years of the last century. Many of today's best writers are as good as any of the past masters. Each reader must find the writers he wants to claim as companions.