Please how can i create web api and mvc solution seperatly ?

Hi all, I need a help..Basically i'm new to the .net application.Here am trying to develop an application for Accounting Purpose.Am totally confused that how can I use the design pattern..MVC is preferring. I have to use this app both in Desktop and as a mobile app.App should be more secure.So please guide me how to design the project

What I have tried:

1.WebApi + MVC is good option i think but for this,should i create 2 solution for both api and mvc?

2. should it work smart phone as well as desktop?

Database-PostgreeSql

Application will have two parts –
Part I – Accessible to the client through the web page
Part II – Back-end accessible only to us (Company) where all the processing is carried out
Perform the initial setup once the client is registered – create the account in the accounting software and create the chart of accounts
1.Review and process documents
2. Accounting – the entries will be passed in the application and will be exported to the accounting software