Center alignment text and image in Twnety Twleve child theme

I am using a child theme with Twenty Tweleve. I have encountered horizontal center alignment problems:

1. On the home page I have an image not in the header that I want to center horizontally on the page. I have not been able to determine the code to enter into my css.style sheet to solve this.

2. On a separate page, text is entered that I want to center. I have been able to do this with text-align: center but then the text is not left justified within the centered position. I assume that I need to add a container first in the css.style sheet, center that container and then place the text within it and left justify it within the container. But I am having problems figuring out how to do this, assuming this is the correct approach.

Help would be much appreciated. The site is tearneywarner.com