In your Settings → Payments section, in the Payment Processors subsection, add the payment gateway you want to use and tick the option Use recurrent payment. Next, set the regular basis when the client is billed per days, weeks, months and years. That’s all that differentiates a subscription from a one-time payment. Payment gateways that support recurrent payment will have this option displayed in the integration box.
Make sure you’ve assigned a price to the subscription in the Assign Values or Formulas to Fields subsection (or different prices to different types of subscriptions) and that the box Enable payments for this form at page top is checked.
For PayPal and PayPal Pro you can also add the number of recurring payment cycles. This means you will limit the number of times the recurrent payment is applied.