Is it a REST API? If so, this should get you started: https://docs.mendix.com/howto8/integration/consume-a-rest-service
If you need to integrate against a REST service, this will help:
https://docs.mendix.com/howto8/integration/consume-a-rest-service
If you need to integrate against a SOAP service, this will help:
https://docs.mendix.com/howto8/integration/consume-a-complex-web-service
https://docs.mendix.com/howto8/integration/consume-a-simple-web-service
If you need something else, please expand your question with an example.