how to use to kentico with Asp.net Web Api ?

Kentico has documentation that you really should search first, along with asking less generic questions and more specific.

For using rest service:

https://docs.kentico.com/k11/integrating-3rd-party-systems/kentico-rest-service

Otherwise just make your web API and use Kentico's API, it has examples all over how to do everything in the api

As Trevor mentioned, Kentico provides you with REST service that implements basic CRUD operations for all content and objects you have in the system.

Sometimes you may want to add some logic into it or format/join data according to your needs. In this case you could implement custom Web API controllers. Here are details on how you could achieve this.