Ask HN: How to improve resume when I am self learned programmer?

I am not actually self learned, my dad taught me BASIC when I was 6 years old and C when I was 8.

But this doesn't fly with HR. My resume is rejected too often, and during tech interviews they often have a very skeptical tone, treating me as of my long career so far been just a con.

I want a job with C if possible. If not, any variant of C is fine. (C++, C#, D...)

But how I prove in my resume I really know C?

Are there any respected Certificates? Online courses? Open Source projects taking newbies or with low hanging fruits so someone can get started?