Before you begin to read from the Torah, you generally answer "amen" to the blessing made by the oleh. That's fine. But remember, your "amen" follows the blessing—it is not a prelude to the reading. A common but wrong practice is when the Torah reader opens the torah, looks at the text, points with the yad and chants "amen" before beginning the reading. This is wrong on two accounts. (1) The only thing out of your lips when the Torah is open should be the words written in the scroll. (2) To say "amen" long after the blessing has concluded is nonsensical.
And there's one other occasion when we often make a similar error. At the end of each of the five "books" of the Torah, the congregation and then the reader chants חזק חזק ונתחזק. The reader must be careful not to chant those words while looking at the text of the Torah if the scroll is still open.