To start with, look through this sticky,
http://forums.androidcentral.com/lg-g4/544443-common-issues-solutions.html , to get an idea of potential issues and how to solve them; it can save you a lot of frustration.
The other thing I can say, given that you've been away from Android for 5 years, is to remember that you're not stuck with the software that comes installed on your phone -- you can install alternative apps, usually better options (my favourites are in parentheses, although there are lots of others out there) for your photo gallery (quickpic), photo editor (quickpic, snapseed), browser (boat, chrome), keyboard (swiftkey), launcher - the piece you use to look at your app list and start them (Nova), dialer, contact manager, file manager (ES File Explorer), brightness manager (Lux), etc.
And finally, if you're going to be using an External SD card, be aware that a) they're not 100% absolutely reliable, so make sure you're backing up your data, and b) many apps are not able to write to External SD, and some won't even give you an error -- some, like ES File Explorer and QuickPic know to prompt you for permission to write to the External SD (in the Common Issues sticky, you'll see the "Editing Photos Save Error" links).
I'm not writing any of that to put you off, they're just things that I wish someone had told me when I started with this phone -- I love my G4 (family first, G4 second, everything else somewhere after that...) and I'm sure it will become your newest best friend