If someone else steals your work, you should be proud. They found it to be valuable. If they managed to sell it or build something with it, they’ve demonstrated that you can do the same. Use it as a learning experience.
Keep in mind that you are in control of what people are allowed to do with your software. By default, your code is unlicensed even if it is public, which means no one else can distribute it or change it or do much of anything with it. Thus, if someone uses it and claims it to be theirs, you can sue them if you want to.
However, instead of leaving your code unlicensed, I would recommend choosing an open source license and applying it to your code when you make it public. There are many to choose from!
By applying a license to your code, you are establishing a clear framework for what other people are and aren’t allowed to do with it. And it’s legally enforceable. In fact, there are organizations that may step in to help you if someone violates your license or challenges it in court. For example, my preferred license is the Mozilla Public License. If someone tried to challenge me on any part of that license, Mozilla would have a vested interest in defending it, since it’s their license and they use it, too. Their lawyer is even available to chat with over email. I once reached out to ask if I could make a small tweak to the license without causing headaches. They got back to me within a few days and said it would be fine. That gave me a lot of confidence to continue using it.
Some licenses are very permissive, such as MIT. Others are much more restrictive, such as GPL. The MPL, which I use, is somewhere in between.
What’s right for you really just depends on what you consider to be fair. And every project can be different. Maybe you build some small tools that you release under MIT, essentially donating them to all of humanity. Meanwhile, you create a startup and build a product where you keep some of it private and release parts of it publicly, licensed under the GPL, because you don’t want huge corporations stealing the work for your day job without reciprocating. That’s a relatively common approach.
Whatever you decide, just make some of your code public. The feedback and experience will be well worth it. Good luck!