If you’re just doing personal projects where nobody else will use the code, then you can make up a package name that you like. Don’t make up something that starts with com.
or net.
or other top-level domain though, because that would imply that you own the domain name (ie. using com.john
as your package name just because your name happens to be John is not a good idea).
If you’re going to give the code to anybody else, you should use a globally unique package name, which according to Java conventions means you should register and use a domain name.