The first, most common mistake is believing that you need an amazing idea to get started. What this usually looks like is someone who believes the key to having a successful startup is starting with a brilliant idea.
The next mistake is exactly the opposite: jumping into the first idea that comes to mind, without really stopping to think critically about whether it's a good idea at all.
The third mistake is to start with a solution instead of a problem.
The last mistake is believing that startup ideas are hard to find. Actually, they're easy to find, because there are many, many real problems left in the world. If you're having trouble finding them, it's just that you haven't yet learned how to do it. Once you learn how to notice good startup ideas, you'll see them all over the place