There are countless email marking companies out there and I've used many of them. I can't say I've come across one that is 100% the perfect fit, but these are my top 2 picks.
If you're looking for an option that's free to start out with Mailerlite is my top suggestion. There are plenty of features you can use that allow you to grow your list without an investment upfront. The only thing that you do have to have is an email address through your domain. Honestly, you should have this before starting an email address anyways. Not only does it just come across as more professional (or legit, because that's really what I want to say), it also helps with your deliverability rates.
The next option, and the one that I use, is ConvertKit. While there is a free option, I recommend upgrading. The monthly fee is minimal and the fact that it's so robust makes it more than worth it! You can create products, landing pages and more. Plus, if you plan to use Kajabi or WordPress, it integrates SO well. As someone who sets up the backend systems and connects different platforms together, this is always a win for me. My website isn't currently in either of those platforms and it's still very easy to create my signup form and imbed it on my site. And can we just talk about the automations that ConvertKit offers?! Oh man, I love them! You can do so much with tags, sequences, and automations which is what you want as you grow.