Classification of skills based on job ads
I don't know about RapidMiner, but for a beginner, Orange seems an awesome open-source tool for the job. 

Particularly if you want to do unsupervised learning, i.e. clustering. You'd probably want to preprocess your textual data in some bag of words model and save it as CSV, I guess.

I also hear Orange has a text mining add-on, but I haven't tried it yet.