How to accept family sharing?

If you've accepted the invitation you received via email to the family group, then just open the Music app (on iOS or Android) or iTunes (on Mac or PC) and select the "For You" or "Browse" tabs to access the Apple Music subscription service.

You don't need to do anything else. Accepting the invitation automatically grants you access.

See here: Get an Apple Music Family subscription - Apple Support

Did you log in with the Family Organizer's Apple ID?

You should be using your own Apple ID, and it should have been invited to the Family Group.