Create an Inferface for different data sources but same datastructure

At the moment I have my sourcedata stored in different excel files but in the future I want to use a database. Since I dont want to write everything new when I have to change the source and maybe in the future there will be other possibilities to store my data so I want to write an interface that I can adjust as I want. The import data is the same so I created a Data-Class (or IData Interface) with the properties firstData and secondData (this names are just for demonstration) and do not depend on the source. My idea is writing a Database_Filler Class and a Excel_Filler Class and the both inherit the IData interface. But I am not sure since I have not done that yet. Could anyone help my get this done?