how to set the class label to datasets?

You have to say what you are trying to do before you can determine what the label should be.
This looks like some kind of text mining project.  What are you trying to predict?

its a kind of prediction project (predict the problem that is frequently ask by user)

So which one of the columns in your dataset is the one you are trying to predict?  That is the one that you need to set as the label using the Set Role operator in RapidMiner.  If the column doesn't already exist, then you would need to create it.