How difficult is it to acquire the skills for playing the notes in the upper register? Thanks.

cf

I recommend the book 'Dr Downing's "Playing the clarinet is easy" By Peter J Moore.
I know it is a bit expensive, but it is worth it for all the knowledge you need
if you are a beginner.

I have no commercial connection with these people mentioned above, apart from being a very satisfied customer

Go to http://www.drdowningmusic.com to find out about it