How can I get Craft Commerce talk to the API of a CRM? A client wants his CRM connected to Craft Commerce. If someone finishes an order the customer data and the ordered products should be send to the CRM.
I assume I have to write a plugin for that, but I have no idea how to start and how the grab the desired data. Can anyone point me to the right direction? The CRM used is https://www.bitrix24.com/ if that's of importance.
You will need to write a plugin to perform the integration. If you make it generic enough, you could share it with the community.

Your plugin would use Craft Commerce events to run PHP code at specific points in the code, which would communicate with the third-party systems.

Docs on event:
https://craftcommerce.com/docs/events-reference

The event you would probably be most interested in is the onOrderComplete event:

https://craftcommerce.com/docs/events-reference#commerce_orders.onordercomplete