At the start of your move phase the first thing you must do is declare which units will charge. Except in a few unusual circumstances explained later, you are never forced to charge. It is always your decision. However, charging is the only way troops are normally allowed to move into hand-to-hand combat. If you want to attack an enemy then you must charge him - you cannot simply move into hand-to-hand combat without declaring a charge. To make this very clear, opposing troops are normally kept at least 1" apart, and troops are not permitted to move closer unless they are charging.
To declare a charge you must indicate which of your units is charging, and which enemy unit it is going to charge. For example, you might wish to declare that your Goblin unit is charging the enemy's Elf archers. When troops charge they move at double their normal speed - this double move is called a charge move.
When you declare a charge you must be reasonably confident that the chargers will reach their target. You may not measure the distance before declaring the charge so you must judge for yourself whether your troops have enough move to reach their enemy. Obviously charging troops must have a clear run at their target. If there are models in the way, you cannot charge through them, and you must take account of any terrain that might slow you down.
Sometimes you may find yourself able to charge an enemy unit in the flank or rear. This is a particularly good tactic as an attack from an unexpected direction could throw the enemy into a panic and may give you an advantage in combat too. A charging unit's position at the start of the movement phase determines whether it can charge into the flank, the rear or the front of the enemy unit.
If the charging unit is in the target's frontal zone it charges in the front. As units generally begin the game facing each other this is the usual situation. If the charging unit is charging from the flank zone it charges into the side; if charging from the rear zone it charges from the rear.
Often the position of a charging unit straddles two areas, in which case you must judge which area the unit is mostly in.
As you declare your charges your opponent declares how the charged unit responds. A charged unit has three response options: stand and shoot, hold, and flee.