Here are three I like:

https://www.quicksprout.com/university/ (free)

https://moz.com/blog/category/whiteboard-friday (free)

http://tractionbook.com/ (chapters 1-3 are free)

Assuming you are just starting out first make sure to get the basics right. What I mean by that is stay far away from these "growth hack" sites or any courses that promises you instant results and hacks. They can be useful but unless you are already getting 500 sign-ups / day you don't need them just now. Save your money because there is enough free information on the net to get some really good results and you'll often find more helpful information in the first 3 results of Google than a $2000 course or coaching.

Secondly, The best approach I have learned when it comes to online marketing is to learn from your competition, esp when you are just starting out. Need a catchy headline? Look at the best adword ads (these guys spend a ton optimizing for clicks), need a great landing page.. Ditto, want to learn how to write the best selling emails, subscribe to all your competitors. You never have to start at square one.

It's useful to start visiting https://growthhackers.com/posts and reading stuff posted there daily. It's like HN for online marketeers.

You dont really need anything to start. Simply do the following:

- Talk to people about whatever problem you are looking to solve. Leverage social media and join in the conversation. Twitter is a good place to find people to talk to.

- Pay attention to the words people use. All problems have a set of specific keywords that tell you everything about how people feel about it. Use these keywords to communicate with people. Talk with their own words.

- Be consistent. Repetition is key. Say the same thing in different words. Repeating is important. Communicate the same message in similar ways.

That's pretty much what you need. Once you've learned more about the problem and those who have it, you can then leverage that data by using tools to reach more people.

Marketing (all of it) is about comminication. Listen and oay attention. Good luck.

Inbound is a great community for SEO related discussions - https://inbound.org

FWIW: If you're asking about marketing after you've built a product, you're doing things in the wrong order. That's why I've included Rob's Start Small, Stay Small as one of core ideas is market validation. From a review: "not about coding but about [finding] a great niche, building a product that executes at a plateau and make it run automatically"

"The other resource I'd recommend is the one I'm releasing in ~3 weeks :)" https://www.julian.com/learn/growth/intro -- julianshapiro | https://news.ycombinator.com/item?id=14026863

Secret Sauce: The Ultimate Growth Hacking Guide "the only actionable Internet marketing guide out there" (2017) $40 https://www.secretsaucenow.com/ -- austenallred | https://news.ycombinator.com/item?id=13807171

Marketing for Developers "Don’t build another software product no one uses. Discover what people will pay for before you start coding." (2015) $39 https://gumroad.com/l/devmarketingbook -- mijustin | https://news.ycombinator.com/item?id=12034104

Traction: How Any Startup Can Achieve Explosive Customer Growth "I started doing a series of interviews on my blog with successful founders about how they got traction" (2015, 2nd ed.) $15 https://amzn.com/dp/B00TY3ZOMS -- epi0Bauqu / yegg | https://news.ycombinator.com/item?id=10346268

Start Small, Stay Small: A Developer's Guide to Launching a Startup "The book is approaching 11,000 copies sold to date, so it's obviously filled a need." (2010) $10 https://amzn.com/dp/B003YH9MMI -- rwalling | https://news.ycombinator.com/item?id=5361601

These guys all use their own techniques so they love bundling additional stuff besides the ebooks.