WPF is a desktop client application technology. It is not intended to be distributed as a service. You will have to rewrite your application so that it is a web application (ASP.NET or some other platform if you don't want to use .NET).
Then you will have to make it multi-tenant so that each client can run it in isolation without interfering with other clients data and you are into the SaaS business. If your application depends on a database, you will have to decide how to make this isolation: whether you will make it at a database level or at the DB schema.
Obviously you will need to choose a hosting for your service. Windows Azure, Amazon, Rackspace, ... are popular choices just to name a few. Depending on your specific needs you might pick one or another.