guns

Ammo brands can have a small difference in residue buildup, but not enough to consider one over another. You'll want to focus more on what shoots accurately and reliably in your specific rifle.

I'd recommend checking out BlackFlag Civilian's videos. He's a leftist who covers relevant topics, including how to select ammo, how to set up comms on a budget, and more.

I'd also suggest TacticoolGirlfriend (who has a great beginner series) and InRangeTV's videos.

I generally like PMC, the 55 grain XTAC is what I stock up on, though I've heard some claim that it's a bit underloaded.

Magtech is pretty alright.

Overrun ammo from Lake City is probably the best bulk ammo, but it's hard to find with so many conflicts going on.

The commercial Winchester stuff is a little dirtier and has an occasional bad primer but it's not horrible. I tend to stay away from this unless there's nothing else.

I've used IMI years in the past and thought it was dirtier than most. I would never buy it again for other, obvious reasons.

I would prioritize accuracy and reliability in your rifle (EDIT: and price!) over how much fouling it produces (the AR-15 can take quite a bit of fouling). This requires testing on your end to determine.

I have never found cleanliness to be an issue with different brands. Anything you buy off the shelf from your local sporting goods store should be fine. Some people have preferences, but you will have to go through a lot of rounds before you notice much difference.

Avoid reloaded ammo unless you are very sure about the person supplying it. Mis-measured powder, or the wrong type, can be very dangerous.

Most importantly for a new owner: have a safe storage space (a proper safe!) and get adequate training and range time with your new rifle.

Not significantly, as long as you aren't buying ammo that's remanufactured, or is some Russian brand (Wolf, Silver Bear, etc.)/Turkish, etc. A bigger issue is going to be the twist rate of your barrel and the weight of the bullet, but even that's not going to be a big issue unless you're trying to get your rifle down to 1 MOA accuracy.

This is your first firearm. Don't worry about the ammo or specifically how accurate it is. Buy in bulk and learn how to shoot it effectively. Put a few k rounds through it first. Clean it, when you get back from the range.

I'd also suggest getting a 22lr upper for it. They're cheap and will let you practice for pennies vs half dollars.