Before you can start texting customers, you need to get their consent. You can approach getting opt-in subscribers in a few ways. If your restaurant has an online reservation scheduler, you can give people the option to subscribe to text messages when they book. You'll then be able to send them confirmation and reminder texts.
If your restaurant does many takeout or delivery orders, you can have customers opt into text messages when they place an order for pickup or delivery.
You can also ask people to sign up for texts when they dine in at your restaurant. Create a form that servers can give customers when they drop the check at a table. Customers can fill out the form to sign up for texts and get access to special deals and news from your establishment.