I quit the company I was working for to pursuit an opportunity as a startup, and I believe in our product.

I'm sure it's going to be great if we attract some customers first to keep going. (I don't want funding.)
Our product is targeted at private schools and courses, and helps organize the mess other LMSs introduce.

The problem is, our team is basically just me and I have very little idea about sales and marketing.

I can do reasonably good copywriting but I'm sure I can do better—and being nervous or too techy in a real world conversation with the client doesn't help.

I want to get better, in fact, a lot better at negotiating with clients and pitching my product.

I did look for some “sales articles” on the web, and a lot of what I found is plain bullshit on SEO-engineered websites promoting books or $5000 courses.

What I need instead is a developer's perspective on how to sale a product you think is great.
What are typical programmer's mistakes and misconceptions about sales, and how to avoid them?
How do you evolve into a reasonably great salesman? I can't believe it's in the mindset and unlearnable.

Your own experience, combined with great articles available on the web is most welcome.

To Future Readers

The question got closed because it is not a good fit for this site.
I found some helpful tips in a similar question asked on a sister StackExchange site about startups:

I'm a terrible salesperson. What can I do about it?