New DM needs help

[GladiusVCreed]

Okay, so I am new to D&D (Have been playing off and on for a year, more off than on)
I liked the game, and so I began to make a spin off using an old D&D Masters Guide and Players Handbook, also a Monster Manual. I am a DM of a family game, and I'm a little confused about how high some AC classes go. I know I should not pit my level 1 family against the likes of an Adult Red Dragon or an angel (I looked up an Angel on the dndwiki). The Astral Deva has an AC of 29? As I have been playing so far (Very much my own style and rules, I based it off D&D but it does not follow it completely) we use a 1d20 on Attack rolls. I know there are things that feed into that roll, but am I missing something on how AC works?

[Honest Tiefling]

Are you adding in Base Attack Bonus, Feat bonuses and magical weapon bonuses? Sir Boris the Strong and Fair, a 14th level fighter, has a base attack bonus of 14. Let's give him a +3 weapon for simplicity, bumping that up to 17. He'll have Weapon Focus and Greater Weapon Focus, bringing us up to 19. Add in a strength modifier of +5 (Assuming he's got 20 strength, which for the level is a mite weak), and then we got an attack bonus of 24.

Or, 14 (BAB) + 1 (Weapon Focus) + 1 (Greater Weapon Focus) + 3 (+3 Magical Weapon) + 5 (Strength Modifier) = 24, which will reliably hit that thing.

Also keep in mind that good aligned monsters are sometimes stronger, because back then, evil campaigns weren't always considered when building. They're not usually meant to be attacked, particularly in 3.5. Don't look at them (unless you have a really fun family that does evil really well together), look at the evil guys for a better idea of what is expected.

[Red Fel]

First off, you probably want to move this to the relevant subforum. The mechanics vary between, say, AD&D, D&D 3.5, D&D 5e, etc.

Second, I'd advise you to stay away from Dandwiki until you have some experience under your belt. That site does a poor job of labeling what is and is not homebrew material. You're better off looking at more standard sides, like the SRD if you're using 3.5. (By the way, here is the SRD's entry on Angels. For reference.)

Third, AC. Assuming you're running 3.5, here is an explanation of AC. Certain parts of AC may or may not apply to a given roll. For example, in a standard "I'm standing in front of you and hitting you" scenario, you'll apply all bonuses to AC. In the Astral Deva's case, that's a 29. But sometimes an enemy is denied its Dexterity to its AC, such as when it is caught flat-footed. In those cases, you remove the Dexterity bonus. In the Deva's case, that's +4; when an Astral Deva is flat-footed, its AC is 25. In still other cases, some attacks - called touch attacks - bypass armor bonuses, such as from armor, shields, or natural armor, but can still be avoided by Dexterity. In those cases, you use touch AC, which is the AC minus any armor bonuses. In the Deva's case, its natural armor bonus is 15, so its touch AC is 29-15=14.

Fourth, CR. You didn't mention this, but it sounds like you're trying to figure out how to balance encounters. The (admittedly imperfect) tool developed by WotC to measure that is Challenge Rating, or CR. Every monster has a CR. The CR is the suggested level that a party of four adventurers would need to find the monster a moderate challenge. An Astral Deva has a CR of 14; it would ostensibly be a moderate challenge for a party of 4 level 14 characters. For a party of 4 level 1 characters, it would be a slaughter. Be aware that CR is an imperfect metric, some encounters are more challenging for their CR than they should be, but generally few are less challenging than their CR.