Unfortunately, it's not possible to create subscriptions or recurring payments with Shopify. This may be possible to set up manually with your payment gateway, however this is not something we support or can assist you with.
As a solution, many shops set up an email drip campaign through their email marketing app. Every month (or whatever time line you set), an email gets sent to your customers reminding them to re-order.
You can find our great email marketing apps through The Shopify App Store.