To add a button to your website, use blocks from the "Form and button" category. You may add one or several buttons, as well as insert ready-made blocks containing text, images and a button.
If you want to add a button to your cover page, use the blocks CR12, CR15, CR16, CR17, CR19A or CR30N from the "Cover" category.
If you are creating a custom design, use Tilda's built-in design editor Zero Block. Learn more about Zero Block: http://help.tilda.ws/zero