When you click on Format, Tableau opens a whole new pane on the left, instead of the Data Source one. The formatting pane is highlighted in this screenshot:

There are five different icons for the various formatting options available. For each option, the formatting can be applied on the Worksheet, the Rows, or the Columns, and on different parts of the View (Worksheet, Pane, Header, Title, and so on). The five formatting options allow you to:

• Format Font: Select the font type, size, and color
• Format Alignment: Change the text alignment, direction, and wrapping
• Format Shading: Add a background color, and column or row banding
• Format Borders: Add a border and column or row dividers
• Format Lines: Change the format (plain, dotted) of the different lines in Tableau (grid lines, zero lines, trends lines, and so on)

With those options, you can quickly make everything beautiful or ugly. The only advice I can give you on this is to keep it simple, readable, and easy to understand!