rules of thumb: how to hit

d&d is a god-killing game. you can kill a god, if you want to. you don't, by neccessity, have to kill a bunch of kobolds first. (fighting all the guy's underlings' underlings, then all the guy's underlings, then the guy, is an unusual progression, and everyone - the players, the dm, the enemy characters - should try to avoid this.) every encounter is appropriate for every character.

to hit: roll a d20 + your attack bonus ('to hit' bonus, initially 0). if the result is greater than or equal to the enemy's AC, you have hit them. your attack bonus should include you STR modifer for melee attacks, and attacks with thrown weapons, or your DEX bonus for missile attacks.

on a successful hit: you roll your weapon's damage die (for swords, d6) + your damage bonus, if any. you deal that many hit points of damage.

An unarmored opponent has an AC of 10. If you have an attack bonus of +0 and you're trying to hit an unarmored guy, you succeed on a roll of 10 or more (55% of the time).

Demogorgon, Prince of Demons and God of Death, has an AC of 27. A character with an attack bonus of +17 will hit Demogorgon on a roll of 10+ (55% of the time). If your attack bonus is somewhere under +17, you should not try to hit Demogorgon.

leather armor is +2AC, chain armor is +4AC, usually plate armor is +6AC, but a full suit of plate armor is +9AC. a shield is +1AC. you add your DEX bonus to your AC.

a d20 roll with a result of 20 on the die is always a hit, a 1 is always a miss.

fighters get +1 to hit every two levels.
clerics get +1 to hit every three levels.
thieves get +1 to hit every four levels.
wizards get +1 to hit every five levels.

or

fighters get +2 to hit every three levels.
clerics get +2 to hit every four levels.
thieves get +2 to hit every five levels.
wizards get +2 to hit every six levels.

monsters get +1 to hit for each hit dice.

fighters get +1 damage at first level and +1 damage every two levels.

attack bonuses:
-when you're not in front of the enemy, ignore their shield bonus to AC.
-when the enemy cannot see you, ignore their DEX bonus to AC.
-get +2 to hit if you're completely behind the enemy.
-get +4 to hit if the enemy is distracted or cannot react. (stunned, prone, grabbed, running away, backstab. also, if they cannot see you, ignore their DEX bonus to AC.)
- if the enemy is helpless (magical sleep, paralysis, prone with no weapon and no shield) you ALWAYS hit them, and you can hit them twice per round, and you deal maximum damage each time.

you should not try to hit Demogorgon with your sword. you should try to hit him with Orcus, Prince of the Undead.