If your store doesn't already include a newsletter subscribe box in the footer you can add your own.
To add a newsletter subscribe box to the footer, click on the menu Design > Theme Settings.
You will now see the Theme Settings screen. On the left hand side you will see rectangular boxes (blocks). These blocks can be dragged to the layout section on the right hand side.
Drag the block called "Newsletter > Newsletter Subscribe" to the bottom left hand side. Drag it into the section called "Widget Top". Set the position to 1.
You should now have a newsletter section at the bottom of your website. You can customize the colours of the newsletter box with the visual editor. This can be found under Design > Theme Settings > Customization and then clicking on Live Customize Design.