I was in your shoes re getting back into the game not too long ago. I would recommend going with points over power level, as from my experience (though other have had different ones) most people play with points, and it makes it more balanced for a pickup game. Use Battlescribe to easily calculate the points and have a printable list. (I can see how people would prefer power level if they had to calculate points themselves. GW did not make it easy to do with the codexes alone.)
For gaming, I would recommend letting the person you are playing against know that you are just getting back into the game and your objective is really to learn. If you can play someone relatively inexperienced too, then that will make a more balanced game and you can both learn. And if you are playing a better/tournament player, then you have the opportunity to learn from someone very good. I think most people like that would be happy to explain why they are doing what they are doing, and how that enables them to beat you. Don't go into the game looking to win, but looking to learn.
One thing that helped me was not really worrying about stratagems during my first couple games. That let me concentrate on learning and getting solid with the mechanics of the game first and not getting overwhelmed. Then once you have a few games under your belt, look through your codex and make a list of the stratagems that you (1) can use with the models you have in your army, and (2) you think will be most effective. Narrowing down the number of stratagems to consider, having an idea of what situation to use them, and having them listed out on a separate piece of paper will make it easier to think of them and use them during the heat of battle.
Don't be afraid to take a moment to think about what you are going to do. And if you have a friendly opponent (which most should be; don't waste your time with someone who is not, or let them discourage you), talk through your strategy with them while you play. Or at least keep a dialogue of what you are trying to do, e.g. "I'm intending to move these guys out of LOS; do you agree that you can't see them?" That keeps everyone on the same page and prevents arguments from cropping up later. Also, don't be afraid to ask questions about the rules! Even though 8th edition is simpler, there are still some tricky mechanics to learn. Especially about moving during the assault phase,
Once you play some games with your army, you will have a better idea of what you should add next.
Also, one thing I did was play in a couple local tournaments. I didn't win any of my games, but doing so allowed me to get a bunch of games in a short period of time, which helped me learn faster. I told my opponents up front that I was still learning the game, and they were all super friendly.
Edit: I would also recommend having a printed version of your army list vs. just using an app. Doing so will allow you to more easily show it to your opponent (handing them some papers vs. your phone or tablet). And I found that it's faster to look at paper than use the app, which takes a bit of time to load and navigate through.
Edited by Aarik, 12 August 2018 - 09:27 PM.