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This is the beginners guide to combat. Let's face it. Combat is an inevitable part of Dungeons and Dragons. Let us first start by explaining Roles.
Roles are the portion of combat that each party member can participate in. Keep in mind, there are also noncombat roles as well.

Melee Damage: Your job is to do as much damage at close range as humanly possible. Take the opponent down before they take you down.
Good Classes: Barbarian, Fighter, Ranger, Paladin, Monk, Rogue
Important Stuff: Strength, Constitution, Combat Feats, High AC.

Tank: Another physical role. Your job is to sit in front of the weaker classes who cannot take as much damage as you can. Often, Tank coincides with the Melee Damage Role.
Good Classes: Barbarian, Fighter, Paladin, Monk (With high Wis),
Important Stuff: Constitution, Strength, Good Fortitude Save, Defensive Feats, VERY high AC and Health

Quick Combatant: You fill a variety of roles. However combat is not typically the specialty of the quick combatant, who are usually more skill based. They often revolve around their dexterity score and light armor in combat.
Good Classes: Rogue, Monk, Druid, Ranger,
Important Stuff: Dexterity, Intelligence (A good tumble check may be necessary), High Reflex save, Light weaponry and Weapon finesse always suits the quick combatant well, Good to make use of the "Feint" combat action. Poisons are nice too.

Ranged Damage: You're a very important backup to the close up fighters, often watching their back from enemies who would approach from behind. You're job is to take your bow, or spells, whatever you're using, and pick off the enemies the close up fighters may not be able to handle on their own.
Good Classes: Ranger, Bard, Rogue
Important: Good Dexterity, Tumble (May need to run away),

Blaster: Spells. Just Spells. Pew pew pew. Lasers. PSSSHHHH. Fire. Whatever you need to get the job done with magic should be at your fingertips. Often the blaster can target multiple enemies at once, making everyone's job easier. Magical items like Holy Water and Alchemists Fire are nice too.
Good Classes: Wizards, Sorcerors, Bards,
Important Stuff: High Spell Modifier, Tumble, Concentration, Spellcraft,

IT SHOULD BE KNOWN IT IS GOOD TO BE GOOD AT MORE THAN ONE TYPE OF ROLE.

Melee Combat

"I strike at toward the goblin with my greatsword!"
Simply put: Hack away, my friend.
Requirements to go into Melee combat:
You are alive and conscious
Your weapon of choice is drawn
You are next to your opponent (5ft=Touch/Melee Range)

Roll. Add modifiers.
Typically, Roll+StrengthMod+BaseAttackBonus+WeaponModifiers+MiscelleneousModifiers

Melee Combat: THE LONG VERSION
Got lots of different options: Tripping, Feinting, Disarm, Grapple, Shoving, Etc. Read the book for details, figure out yourself when is the best situation to use each one.
There you go.

TO BE CONTINUED

It says to be continued doesn't it!? Gawd.