How to stop the player from moving on. On till they have done the objective.

I am making a horror game that is set over three levels witch are just the forest getting darker and eviler. On the first level i want it set up so that the player has to pick up info before they can move on to the next part but i can not work out how to do it i was thinking of making a invisible wall that moves when they have got them all but then how can i make it move on to the next level. ??

Sorry for my bad English

Have you checked out IF statements?