Welcome,
the most efficient way will be to start with any project you like to realize,let's say a self guided tour through your city. You will start to design your app. You want to have
- a map with points of interest
- a list with points of interest
- Information pages
- some material need to be downloaded from the user
- ....
the next step is to look to the components
- map: you will need a lib, you will search the forum and/or documentation: OSMdroid or googlemaps lib
- a list. no Lib needed
- Information page: no lib needed
- some material need to be downloaded from the user: you will need additional classes/Lib to start the download...
- ....
so I think, the best is to start with a project
Good luck
Stefan