At the start of your Movement 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 which units, if any, will charge the enemy.
However, charging is the only way that models are normally allowed to move into close combat. If you want to attack an enemy then you must charge him - you cannot simply move your model into close combat without declaring a charge first. All charges are declared at the start of your Movement phase, in any order you wish.
To declare a charge, indicate which unit is charging and nominate one enemy unit that it is going to attack. For example, you might declare that your Empire Knights unit is charging your opponent's Orc Warriors unit.
A unit may only declare a charge if at least one model in the unit can see at least one enemy model in the opposing unit. It is not necessary for every model to be able to see an enemy - the whole unit is able to charge the enemy so long as one model can do so. For line of sight see.
Troops can only charge up to a predetermined distance - this is called a charge move and is explained later. When you declare a charge you must do so without measuring the distance to the target, you must rely on your estimate of the distance to ensure that your troops can reach their target.
When deciding whether to charge you must also take into account any terrain that might slow you down, as described later. Deciding whether to charge or not calls for a good judgement of distances!