Dictionary API for spellchecker

Which is the best way to create an API for a desktop APPlication in VB.NET that fetches a dictionary from a web API? How would you go about creating this system? The only thing that am finding online is how to create an API for a website or web a page to load data from a database. What I want is how to create an API to store a dictionary to retrieve that dictionary from a program connected to the internet? Is there a way to prevent users who have not paid from using the desktop software?

My tools developing the program

1. Visual studio 2012 for web (So, I have ASP.NET, and am familiar with MVC!)
   
   2. Visual studio 2012 for desktop Applications.

Currently, my grammar checker program is working by the use of a dictionary from the desktop! How do I make this software available to other users after they have paid to use the software? I cannot afford to create a website where they will be doing it from a web form; the web form is for registration and paying only to download the software. This cannot be a one time fee, they have to pay every month to use it!