How to setting up desktop navigation software ?

Hello I am trying to program desktop software for the navigation system. I already created software in Visual Studio (C #) with the Dotspatial library, but it has a lot of problems with performance and so on.Right now I have decided to build software like openstreetmap that runs locally and is not dependent on the Internet. So I read articles on this and was able to download osm files and then upload them to my local database system (Postgres with Postgis plugin).I used software "osm2ogsl" for this. Now I need to learn how to retrieve data from my local database and render it as mapbox or mapnik renders and then display.I try to program in Qt framework. I need a clear roadmap. Thanks