Step 1 is to understand the tool you're using and manage expectations.

A basic milspec AR in 5.56 is not a precision instrument. I've seen them shoot sub MOA, but most shoot MOA or slightly worse. So, start with the expectation that you'll be seeing groups no better than 2" at 200 yds and be pleased if it's better.

Step 2 is to decide if you want to relegate the rifle to target/varmint use with a high power optic, or if you want to keep it closer to it's design intent and have something that is also useful as a HD/bugout gun.

For the former, pick the high power optic of your choice. The best advice is usually to buy the best glass you can afford. It's not uncommon to spend more on glass than the rifle.

For the latter, any of the 1-6/8x LPVO optics designed for the AR would be a good choice. The allow you to use it as a red dot on 1x up close, and dial up the power for precision shots at longer distances. I have a Vortex Strike Eagle in 1-6x on a 16" AR, and it has shown MOA performance on paper out to 150 off the bench, and reliable hits on steel out to 300 in timed drills. IMHO, it performs pretty well for the price point.