Items in the center pane can be sorted by various fields, such as their title, creators, or the date they were added to your library.
To change the way items are sorted, click on any of the column headers at the top of the center pane. For example, if you click on “Title”, all your items will be sorted alphabetically by title. Clicking a header multiple times toggles between ascending and descending sorts. (The header will show an upward and downward arrow, respectively.)
By default, Zotero will show columns for the Title, Creators, and Attachments properties in the center pane. You can change which fields are shown by right-clicking on the column headers and selecting fields from the drop-down menu.
For each column, you can also choose the Secondary Sort field (the field that's used to break ties when sorting).
By default, fields in the center pane are arranged from left to right in the order in which they are shown in the drop-down menu. You can rearrange them by dragging and dropping the headers. To reset the order, select “Restore Column Order” in the drop-down menu.