Lots of things make startups hard... building teams, shipping products, finding customers, earning revenue... to name just a few. But when there's an endless list of things to do what takes priority? I'd argue it's getting good at learning.
When you start a company, you may have many ideas but you don't have any proven answers. You don't know who your customer is going to be, what product you're going to make, which features are going to matter, what your customer will pay, and so on. Therefore, the most important thing to get good at is finding answers to all these questions.
How do you do that? You get really good at learning.
Luckily there's lots of ways to learn rapidly. Get in front of potential customers early and often. Iterate on design, prototypes, and product continuously. Collect data by using your products, watching others use them, collecting quantitative and qualitative data, and acting on what you see.
To get everyone excited about learning, let them share and celebrate their discoveries. My favorite way of doing this is with a regular "what did we learn this week?" meeting that allows designers, researchers, engineers, and more to highlight what they found out that week about our customers, products, technologies, etc.
This simple process only takes an hour (do it over lunch) and goes a long way to improving how well a company learns.