Hi there
Well done on deciding to book your first JY holiday, it's the hardest part trust me - I dithered for months but so glad I did it.
I've not done either of the Croatian trips, but based on my 3 other JY holidays you'll get an information pack nearer the time of your holiday detailing your flights, hotels and all of the optional extras and their prices. Sometimes you can prepay for the extras when booking your holiday, other times you pay the Tour Manager when there. Based on my experience, I'd say that in most cases the optional extras can be done cheaper yourself if you're prepared to do a bit of research beforehand - the downside of course is that you'll lose the use of your Tour Manager and won't travel with the rest of the group. Personally if any of the extras interest me (and most of them usually do!) I've always paid and gone with the group - the Tour Managers are very knowledgeable and without doubt make the trips more enjoyable and interesting as a result.
The hotels have always been the ones listed on my 3 trips, don't know if that's always the case though - sorry!
Hope this helps,
Michael.