When to register for self assessment?

I am just starting as a "sole trader", as a Yoga teacher along with my PAYE job. I've got a couple of questions. I am a little unsure as to when to register. Do I wait until this tax year finishes in 2025 and then register, or do I do it sooner? I have searched through many pages here, and other Internet sources but I am unable to get a confident answer to this. I am also probably registered as Special Section D. I do not need this, and it was placed on my record automatically in 2015. Is it worth me getting it removed if this is the case? Thanks in advance.

Hi,

You can see guidance here:

Set up as a sole trader

Thank you.