Do the usual ->settings->about phone
Then tap Build Number till it tells you are a developer.
Then go back:
Now select [] Developer options
Turn on debugging.

Now connect USB cable and "USB computer connections" should then be available to make selections.

Actually if you want adb shell to work you must unselect all options.

Then a pop-up asking if you want to authorize such and such computer access, Say yes.

And adb shell should work.