I'm looking into getting an AR-15, this would be my first firearm. I'm wondering if I should be worrying about what brand of ammo I choose to purchase, will it affect how quickly the barrel becomes dirty etc?
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.