Web services API of CRM online

Dears,

please I need your help in this matter.

I've done with creating customize entity in CRM. Now I need this entity to be accessible by another party (xamarin).

I found that it can be done through web services Api.

My Question is that, from where I can generate this API (how I can generate CRM API?) so the entity can be accessible to be read, write and update from third party.

thanks ,