web api rest way with re-direction

See more:

C#

.NET

web

REST

API

Hello,

I am working on new projects where our requirement is to get functionality using web api rest way.
Here is overview of how system will work.

Client makes first call to our system
System will check for active session if not it will redirect to login page
Once logged in it will get some information from user
The more information
Do some call to third party api
Do some business logic
Finally reply back to first call with some useful information to client so client can do their work on the basis of response.

Could you please suggest what would be the best way to implement it using rest webapi with JSON format so clients from various platform can use this flawlessly?

I appreciate some inputs.