If you're serious about playing Age of Sigmar, then you'll need a copy of the Generals Handbook. It is where you'll find the points for all the units, grand alliance allegiance abilites (you can always use those instead of buying a specific battletome and using those), and it contains all the missions.
Like any game, you need to pay missions to get the most out of it. If all you play is line up your armies until last man standing, you'll never enjoy the game, or any game.
Daemons have some good variety of units. All the ones Khorne has are solid, so you're in luck. Bloodletters are phenomenal, but only in large squads. You'll want to run 20-30 man units as your battleline, and they'll threaten any unit in the game.
Skull cannons are also a great unit, as they are your only source of shooting,and are a great unit to try and snipe support characters with.
The one thing about the army, is that it's not terribly durable. You'll hit hard, but you can't take a punch. You'll need to carefully consider your order of units you pick in the assault phase, as you might loose units before they can attack.
It's also not a particularly fast army. Not allow either, but it's easy for the bloodthirster to get out ahead, isolated and killed. Keep it screened until you have a good chance to strike.
Also, seriously consider picking up the mortal Bloodsecrator it's an amazing force multiplier for all Khorne units.
The bretonian units I don't know much about. Them, tomb kings, and a host of other older models have been phased out of the game, so I've not played much attention to them.
You can still use them though, and they have updated rules and points here.
New and Updated FAQs, Forge World Warscrolls, and Compendium PDFs - WarhammerYou're totally welcome to keep using them, but keep in mind they'll never be updated in the future and you'll not be able to buy new models. I'd strongly consider finding ways to convert them so you can use them in existing warscrolls. Maybe as part of Free People's.
I'd also make sure you read the rules, and print out the warscrolls off the GW store (or use the AOS app), and learn what your dudes can do.
I can't help you much in finding opponents, but if your local gaming store has a Facebook page, you can always reach out on there.