Learn how to field strip and reassemble the Ruger SR-556 piston rifle from a Ruger professional. It’s very similar to most other AR-15 piston clones. It’s much easier to learn from a video than a gun manual, for sure.
Always use a good gun cleaner and gun oil in order to make the most of the AR-15’s reliability and accuracy. I highly recommend M-Pro 7 Gun Cleaner, Hoppe’s Elite Gun Cleaner, Mil Comm MC25 and Slip2000 725 Gun Cleaner. As far as oils go, I recommend Rem Oil, M-Pro 7 LPX, Breakfree CLP, Ballistol, Hornady One Shot and Hoppe’s Synthetic Gun Oil…just to mention a few that did well in my gun oil testing.