Add API to existing Laravel project

Saying API is a very broad thing, perhaps you could be more specific as to what you are trying to accomplish.

I need to allow user to use my site services via mobile app

Is it an app only for mobile devices, if not you can make a regular site mobile friendly with media queries and mobile detect software. Still don't know what you are after. Edit: see if this helps: https://laracasts.com/discuss/channels/general-discussion/web-mobile-applications-services-architecture

Thanks jlrdw, I have read that topic but didn't solve my problem. Already exists a mobile versione of the site with media queries but the client required me to develop native app, so I need to create a api to connect the mobile app to the site ( login - registration - profile - CRUD )

If I divide API and web routes should I write two times my application and I think that in this way I can miss some functionalities but if it the best way to do it I will.

Any other option?

I think ideally you'll want dedicated api routes that use similar functionality. What that might look like in practice would be stripping logic out of your controllers into reusable traits so that your controller methods simply call the appropriate trait method to get data, and the controller responds either with a view template for your existing routes or with json for new api routes.

As for authentication, you'll likely want to use something like JWT