One of the most important aspects of any game system is task resolution. This refers to how the GM and the players resolve the actions that the characters attempt to perform.

The basic idea is that rolls should only be called for when required, or to increase dramatic tension. If the player wishes his character to perform an action, and the character has the required skill, and there is no undue pressure for the completion of the action, then it should succeed eventually, with no rolls being required.

However, should the character be in danger or under some sort of pressure (time, adverse weather, etc.), or the GM wishes to heighten the dramatic tension of the scene, then he should ask the player to roll.

Knowing when and when not to call for rolls is one of those things that a GM can only learn through experience. Keep in mind the advice just given, and that should lead you in the proper direction.