There are basically two ways to do this, depending on whether or not you've set a width for the column. They both involve setting the column's left and right margins to the same value; it's just a matter of what value you give it.
If you've set a width for the column (in pixels, points, percentage, or anything else), then set margin-left and margin-right to "auto" (no quotes). This will put enough space on each side to center the column.
If you haven't set a width for the column (so that it will be as wide as it can possibly get), then set margin-left and margin-right to however much space you want on both sides. All that matters is that you set them to the same value; as long as you do that then it can be anything you want (px, em, %, or whatever). You can also play with this to make your column be slightly (or even greatly) off-center, by setting margin-left and margin-right to different values.