Before you can get an NDIS plan, the NDIS need to know some things about your diagnosis and how your disability affects your life. There's a bit on the form they'll give you that your doctor can fill in, or you can give the NDIS copies of reports you've gotten from medical professionals.
The NDIA needs two things from your evidence: your diagnosis and how your disability affects your everyday life. That second part , the functional impact, is where many applications succeed or fall short, so make sure your reports cover it clearly.
Good evidence usually comes from the professionals who know you best:
Recency helps too. Reports that reflect your current situation carry more weight than ones from years ago.
The NDIS website has detailed guidance on disability evidence.