The minimum role required to create alerts is member. Sentry users with manager or owner permissions can change the minimum role requirement in Settings > General Settings > Let Members Create and Edit Alerts.
To create alerts:
Navigate to Alerts and click "Create Alert Rule".
Click "Set Conditions".
On the alert configuration page, set the conditions of the alert:
You can also create an alert by duplicating an existing issue or metric alert rule. To do so, navigate to Alerts and click "Duplicate" in the context menu (under "Actions") on the row with the alert rule you want to copy: