You'll find rules for Mounted Combat in the Dungeon Master's Guide, pages 76–77. Let's elaborate on them a bit.
Jousting[]
Jousting is a type of combat where two mounted warriors, armed with lances ride at one another and try to spit one another with their weapons.
It's the sport of knights and kings, and is very appropriate to campaigns with a medieval setting.
Initiative[]
As a special rule, when two riders are jousting, don't bother with initiative rolls. On the first round of a joust, the two jousters ride at one another and roll attack rolls; on the second, both continue their ride, turn around, and start back toward one another.
All attacks are resolved simultaneously. Initiative does not enter the picture.
If jousting is taking place during a larger battle, where other things are going on and initiative must be rolled, try this: When it's time for the jouster who won initiative to take his action, the jouster who lost initiative also gets to make his attack roll, though this takes place during the middle of the enemy's combat sequence. This helps simulate the fact that the lance-strikes take place simultaneously.
Multiple Attacks[]
When a character has multiple attacks in a single round, and is jousting, he does not get multiple lance attacks. He can only use that lance once in a combat round (and loses the next round turning his horse, if he's to come back and face the same enemy).
However, that second attack in the same round can still be effective; the best use for it is for the rider to Parry the incoming lance attack with his shield. This can make a joust a lengthier affair—which isn't a bad thing; a classic joust should go on for a few rounds at least.
Lances and Dismounting[]
The DMG, page 76-77, talks about being dismounted in combat, but doesn't discuss at length the usefulness of the lance in dismounting opponents.
The lance is designed to dismount opponents as well as to inflict damage. Anytime a lance hits a mounted target and does 8 points or more of damage (after doubling), the victim has a chance of being dismounted. He must roll his Riding proficiency; if he fails, he falls for an additional 1–2 damage.
Since both lancers can hit at the same time, and both can conceivably miss their Riding rolls, both can be dismounted simultaneously . . . which tends to be embarrassing, and can be dangerous in large battles.
Lance Breakage[]
Lances often break. Any lance that hits and does more than 12 points of damage, and any lance that has been successfully Parried by a shield, may break. The player rolls 1d6; on a 1 or 2, the lance breaks and is useless (except as a club).
Lance Specialization[]
If a character takes Weapon Specialization in lance, he gets the usual benefits of attack and damage bonuses. But he also gets the following benefits:
He can perform the Shield-Rush maneuver with his lance-tip. In order for this to do damage like a Shield-Rush instead of a lance strike the character must be mounted, and must strike a character who has metal (or leather-and-metal) armor or a shield. If the character does not have such armor, the attack is automatically a lance strike.
He gets a +2 bonus to AC (thus, an AC 2 would become a 0), only when jousting and only vs. another jouster. (That is, the AC bonus is only against incoming lance attacks, and only works when the defender is himself mounted and carrying a lance.)
Horseback Archery[]
Only the Composite Short Bow, Short Bow, Daikyu (from this supplement's Equipment chapter), and Crossbows may be used from horseback. The Long Bow and Composite Long Bow may not be.
Firing a bow from the back of a still horse (one which is not running) is done at a –1 penalty to attack rolls.
Firing from the back of a walking or trotting horse is done at a –2 penalty to attack rolls.
Firing from the back of a galloping horse is done at a –4 penalty to attack rolls.
In some barbarian tribes, especially those who inhabit wild-open plains, every warrior knows Riding for the tribe's favorite riding animal and Specializes in Composite Short Bow. This makes them very, very dangerous opponents in a cavalry engagement.