Existing Web App - create an API for customers - how?

I have a asp.net 4.5 webforms app with a SQL Server 2014 backend

Customer(s) wants to access his data via an API
We currently don't have an API so need to create one

The web app uses 3 tier's:
1 Web
2 Business Logic
3 Data Access Layer

Would I use the business logic layer for the API or just deal direct with DB?
Can someone give me an idiots guide on how to get started using .net, and rough outline of architecture
I also have no idea about securing it?
Any advice appreciated on how to get started speccing it out
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käµfm³d 👽Commented:
Would I use the business logic layer for the API...

...or just deal direct with DB?

While you could expose some web services via Web Forms (crudely), or even through old ASMX services, I would suggest going with Web API. I find it a bit easier to write. If you stick to CRUD as best you can, then writing the web service should be straightforward. You can find details on writing Web API on the official ASP.NET site.

As far as securing it, there are multiple approaches. Basic authentication via username and password is one way--just be sure to operate over SSL so that usernames and passwords aren't send in clear text over the wire. You could also do certificate-based auth or even token-based. This would be a decision you need to make. The ASP.NET site should also discuss security. I believe Microsoft is now pushing their ASP.NET Identity for security. I haven't used it yet, so I cannot really comment on it.

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