Hi Francois and Welcome!
From what I read I think you are using Cantabile 3 Lite. Is that correct? If so then we need to know how much you have already done. If you could explain how much you have working it would help. From your description of what you would like to do I don’t think that the Lite version will do the job. You might need at least the Solo version to use the onscreen keyboard as a good MIDI controller. The Lite version does not have this capability. A good post I spotted today by Torsten Ecke gives a good summary of how to start learning the Cantabile 3 product. As he indicates, the software is like a modular synth so learning it can take a little patience. Here is a link to @Torsten’s post :
Thanks for posting
Dave