To start out, you will need to create a new Incoming Webhook, you can do this by navigating to Custom Integrations and search Incoming Webhooks, or click here
Choose the default channel you'd like to send messages to. You can change this channel in your options, or per-action. We recommend setting this to #general by default. After you select the channel click "Add Incoming WebHooks integration
On this next page, make note of the Webhook URL, you will need that later. This page can be customized as needed, but leaving everything as the defaults should be fine.