What happens to MY music if I join a new Family Sharing account
Family Sharing allows all members to access Apps, Books, Tunes etc purchased by all members [though not the 'data content' of any App, so for instance each member's level on Angry Birds, or whatever would not cross-pollinate]. It will also add a shared calendar for which usage is optional.  

It doesn't share anything else. It also doesn't delete anything, only adds.

If your iTunes Library contains CD rips or music not purchased directly via your iTunes account, that would not be available to other family members, you would have to copy it to their Mac/PC/iTunes manually.