I have written a c# application that I want to convert to windows store app and I know they are very similar but I absolutely have no idea how to start and what to do. Any ideas please?
It is simply not possible to convert until it is a portable .NET application/library.
WPF applications and Windows store apps are of completely different beast.
If you want to migrate Windows Store Apps: A Guide for WPF and Silverlight Developers is a good starting point.