Tried for hours to find a way.
Google Play. But many of them are written for devices with more memory, so they won't run very well.
Do you have to sign in with a email to get to plastered?
With a Google account - but they're free. Just set the tablet up in Accounts for a New account, a Google account, then when it asks new or existing, choose new if you don't already have one.
How do you get rid of preinstalled games?
You're running Linux. (Android is a program that runs in Linux.) It has no way to get root permissions, so you have to root the tablet.
How do you transfer to external cards?
Games? I wouldn't suggest it. It can be done, but Android wasn't written for it, and many apps and games that will transfer won't run from the card. Use Application manager, find that game, look for the Move to SD card button and, if it's not grayed out, tap it.
(From another senior.)