Yes, I know it's an old post, but I think its worth while posting the actual procedures that have now changed.
All SCC personnel are volunteers
If you just want to pop up once in a while to help run the galley at stand easy, help with the boats or other job you can be a Unit Assistant but most join as a PCI (Probationary Civilian Instructor), nomatter what (unless you were a Petty Officer Cadet/Cadet Sergeant then you would join as a Probationary Petty Officer/Sergeant).
After 6 months, and completion of an Adult Induction Course you can apply to become a UNIFORMED member of staff (Acting PO/Sgt) or simply stay as a CI.
There are core modules that need to be undertaken when going into uniform (Drill, Instructional Methods, First Aid, Seamanship, Child Safety) to be completed within 2 years then you are promoted to a substantive PO/Sgt.
Only then if you wish to become an Officer, you have to have recommendation from your district and area and then go on a POAC (Potential Officer Assessment Course) then after successful completion of passing that, do an OSB (Officer Selection Board).
Then if you are under 21 you will be a Middie and over 21 a Subbie!
Simples!!
P.S. - The system is way too intense - Joining the RN as an officer seems more simple!