By default, Thesis uses a three column layout, with the content on the left, and two sidebars on the right. You can easily change both the number of columns, and their order, from the Thesis Design Options subpanel:
Expand the Columns section, and choose the number of desired columns in the dropdown selector; then enter your desired widths for each available column:
Note that if you’re using the Thesis Upload Header Image feature, the widths you designate for your columns are used to compute the optimal width for your header image, and so changes here may have an impact on your ultimate site design.
Once you’ve chosen the number of columns, expand the Column Order section, and make your selection from the available choices:
Choosing the option for one column does not eliminate sidebars from your site; instead, they are placed below the content area, within a single column layout. To remove sidebars completely, you would use the thesis_show_sidebars filter.