Items You Need
Dungeons &Dragons contains a dizzying array of magical items that your character can carry and use, but often you'll have to choose which to take with you, and many of them are borderline useless. A handful of magical items in particular are important for every character to have. Try to get both an enchanted weapon and enchanted armor. The Cloak of Resistance will give you a bonus on all saving throws, the Ring of Protection will give you a deflection bonus, and the Amulet of Natural Armor will give you an AC bonus, each equal to the item's enchantment level.
Flanking
Flanking is an important tactic to use when engaged in hand-to-hand combat in versions of the game after second edition. If you engage a foe with a melee attack and a friendly unit is threatening it on the opposite side or corner, your attack receives a +2 bonus. This rule reflects the disadvantage someone fending off two attackers at once has in real-life combat. Working together with your party-mates to flank enemies can help you bring the monsters to a speedier demise. Some foes, such as the undead, are immune to flanking attacks.
Attacks of Opportunity
An attack of opportunity is a free hit you're allowed to make on an enemy who drops his guard briefly during combat to do something else, such as drink a potion, cast a spell, or make a ranged attack such as shooting a bow. A successful attack of opportunity can cause an enemy wizard's spell to fizzle, making it a very effective technique. You can easily provoke attacks of opportunity from enemy combatants as well, however. To play it safe, avoid taking non-combat actions and never move more than the safe maximum of five feet per turn, when in a fight. If you're a spell caster, always cast on the defensive which does not provoke an attack of opportunity.
Focus Fire
Most monsters in Dungeons &Dragons don't become any weaker as they are wounded, and can still attack with full ferocity until they draw their final breath. Because of this, it is always a better choice for a party to focus on one enemy at a time when confronted with multiple foes. This will usually take approximately the same amount of time as attacking all of them at once --- but as each baddie dies you'll take that much less damage each turn, rather than receiving the full onslaught until the very end.