This video by Thomas Grainger is really helpful if you need help getting started.
Most people prepare 1 creative then just refine that creative over and over throughout the year until trials and HSC.
Some people prepare 2-3 creatives.
It's up to you.
As for using a specific question/stimulus for your prepared creative, I don't think its necessary. Essentially, you just need to test if your ''generic'' prepared creative can be adapted/moulded to various past HSC and trial questions + stimulus.
It's good that you plan to start now. I remember properly starting my creative in late Term 2 (trials was during early Term 3) - after ditching about 2 previous creatives ideas/attempts during Term 1 - and barely refining it during the holidays before trials. Then got 9/15 for the creative section for my trials
which was my worst mark by far in Paper 1 for trials (got 15/15 for section 1 and 13/15 for section 3 so the belonging essay) and the creative section most likely ''dragged me down'' in the HSC paper as well (even though I still got a band 6 for Adv Eng, 91 HSC mark, creative writing was definitely a weakness of mine).
It's important that you keep refining your story or stories and get it done early so you have more time to refine (trials will come quickly).