First, you may want to create templates. You can do that by going to "Settings > Languages and texts > SMS template > Add new template". On the right side of the page you will see several codes. You can use these codes to add relevant guest and booking information to your messages, that will personalize it to suit the guest you are sending it to.
After that, you can activate automated messages by going to "Settings > Languages and texts > Automated messages > Edit". Choose which of your templates you want to be automatically sent out. You can choose to send them out X amount of days before or after check-in/check-out, or after the booking was made. New booking requests, confirmation and cancellation emails and texts, can also be created as templates and sent out automatically.