I want to use tomtom web api for routing : https://developer.tomtom.com every time I search how to use an API with django I get the restfull framework isn't that for building an API? If I can use it to work with an existing API if yes I want to know how? Or the requests library for python would be enough?
You should definitely try requests
package (link), it is totally sufficient for building any REST API client.
The Django package you've probably found is django rest framework
which is used to build API interfaces. But what you want is not to build an API but make requests and integrate to and external API. Requests package is simple choice to do that.
I have found nice tutorial to API integration with python here. Good luck ;-).