First things first, I think before selling, my understanding of sales was largely grounded as an "art", and by far the largest turning point in my understanding has been that sales has a pretty large "science" component to it as well, by which I mean tried and true repeatable steps that can be applied consistently.
As a founder, before sales, you need feedback. Assuming you don't have product-market fit (which you almost certainly don't know, because you don't have users), it is MOST important that you talk to customers to get a handle on what they need. The two books I found most valuable here are: - The Mom Test - Talking to Humans
(The Lean Startup has a good section on this as well; if, like me, you haven't reread it since starting a company - I would highly recommend doing so, you grok so much more than when you first read it without any experience to ground it to.)
Once you have some intuition around your market and that your product is actually tackling the right problem. I would read Founding Sales (https://www.foundingsales.com/). I was recommended this book by a founder-friend who told me "Literally stop whatever you or your team is doing and take the next two days to read this book" and I'm glad I did that. The book has given me a solid anchoring, vocabulary, and makes a compelling case that founder-sales are fundamentally different than regular sales: regular sales are what happens AFTER you have a repeatable process, founding sales requires much more of a product mind to rapidly integrate feedback and live-iterate on the product.
From there, you'll realize you have two skills you really need to learn: marketing and sales. Broadly: marketing is about getting leads, sales is about closing them.
Marketing: There are a whole bunch of books and articles I've been recommended on marketing (Traction being one of top recommended ones). However, after having done a lot of this I'm not convinced that this is a good use of time as a founder. So much of what I've heard / seen is that your first few customers WILL be warm intros from within your network. So a better use of your time may be a LinkedIn subscription and to go through the network of: your VCs, any former companies you've worked at, any school you graduated and shamelessly ask for intros. A good article that popped up on HN recently was: https://stripe.com/atlas/guides/starting-sales. (Also, for the theory of marketing, I personally found Crossing the Chasm a solid book to contextualize what phase of selling I was in and how it might change over time.)
Sales: Is hard. The best advice I've received is: remember it's about them. Not about you. When someone is gracious enough to take a phone call with you, do not immediately pitch them. That makes it about you. Instead, ask them about their process and the pain point you address. This helps both: you discern if it's a real painpoint, them realize that they have / the magnitude of that pain. Two frameworks that have helped me are: - BANT: https://blog.hubspot.com/sales/bant - MEDDIC: https://www.lucidchart.com/blog/close-more-deals-with-meddic...
Also, something that's been really helpful here are mentors. Finding other founders who have successfully navigated from seed -> series A usually have good advice on how they did early sales (also, some sales people that have been at early companies have been particularly helpful, especially when they first acknowledge founder sales is a similar but different animal than they may be used to).
Other than that, there's only so much you can read, just go ahead and try it! I believe that a lot of this probably takes practice above all else, and you're not going to sell your product without talking to anyone!
Best of luck! I'm happy to talk to you about your product - and thank you for posting a question that's been on my and my cofounder's minds for weeks now. And, as I said at the top, I am not an expert--just another person trying my best to figure it out. I've already found a bunch of the other responses useful :)