You don't have to be the world's greatest authority on something to be someone else's "expert." If you're just a few steps ahead of them and you can clearly communicate your knowledge, you become that person's expert. To stay their expert, keep learning and keep sharing.
Or you can be the expert who takes a huge river of information and narrows it down to a manageable stream. You can also be an expert who brings multiple sources of information to people's attention. Maybe you are deeply knowledgeable about one topic. Or, you're a jack of all trades. Or you don't know anything, but you know people who do and can call on them when you need information.
There are many forms of expertise, and if you know something about a topic, chances are you are someone's expert as long as you are willing and able to share your knowledge in a way that they can understand.