How to set up Safe Zones
Safe zones are important to make sure that your title or other important elements in your image do not get cut off. To create a safe zone, simply select View > New Guide Layout.
In the pop up window you can manipulate the measurements of the safe zone. Make sure that everything important stays within the inner rectangle. The outlines won't appear in the saved image.
How to make sure your title is legible
This highly depends on the font you are using, as well as the colors and contrast. Just zoom out by pressing command and - on your keyboard until the image is thumbnail sized to check.
How to Export your images
First make sure that all your images have the correct aspect ratio by selecting Image > Canvas size. You can select measurent units in the drop down menu. If the aspect ratio does not match you can change the size of the file here, however, you might have to edit your image some more to avoid white borders. If your image does not have the right aspect ratio at all, you should crop it instead of stretching it.
Now select File > Save As > JPEG. Select Maximum Quality and click ok.