To side load apps I use adblink on the pc. Yes you can run showbox but there are far better that don't use torrents, Cyberflix, Morph tv and Cinema HD to name just a couple.
The first thing you want to try and get is Downloader and a browser using the search funtion. Another thing I would do is youtube installing filelinked on a 4k firestick, this allows you to enter a code to access peoples download pages usually full of useful apps if you need a code let Me know when you get filelinked on it.
ps If you do get adblink woking there are apps posted here
Android Apps + Software by
@alimac quite frequently.