Attacking
In combat, characters add a modifier, based on a relevant skill, to their d20 roll. For basic weapons attacks, this is the strength modifier. For powers, check its description for the relevant modifier. This number (roll plus modifier) is compared to a target number. For most attacks this is going to be armor class. For spells and other powers, it could be fortitude, willpower or reflex. If the number that the player rolled is higher than the number for the target, then the player has succeeded and gets to roll damage. Each weapon and power will specify the number and size of the die (or dice) that need to be rolled. The result is subtracted from the target's Hit Point total.
Move, Minor, Standard
During combat, each character can take three actions, one of each in the following categories: move, minor and standard. A move action means that you can move your character a number of squares up to your speed. Running and charging are other move actions with specific rules. A minor action might be loading a crossbow, taking out a potion, or lighting a candle. A standard action includes all attacks and powers (unless otherwise specified in the power's description). A player may also choose to make two move actions, or two minor actions and a move action.
Skill Tests
There are 17 character skills in D&D. These skills have a score based on whether the character has chosen to train in that skill and the modifier of the ability that the skill is based on. Training adds five points while the modifier will continue to rise as the character gains levels and ability points. When a character needs to use one of these skills, she rolls a d20, adds the skill score and compares that number to a target number. The target number is set by the dungeonmaster and is based on the difficulty of the task, plus any environmental factors the DM may feel are relevant.
Death, Dying and Saving Throws
A character who has been reduced to zero hit points is not dead. He is unconscious and must make a saving throw to avoid death. A saving throw has a target number of 10. If the player rolls a 10 or higher, he or she has succeeded. An unconscious character must continue to make saving throws against death every round until he can be healed or until the combat ends, whichever comes first. If the player fails the saving throw three times, then his character is dead. Dead characters can be resurrected by certain priest spells and rituals if there are mortal remains. Characters are sometimes required to make saving throws to end spell effects. If he fails, the effect continues to the next turn, when he may try again.