Sometimes if you don’t specify that your character is white, people will default to their own ethnicity.
Also, there is nothing wrong with having white characters if that’s what you’re comfortable writing. Diversity doesn’t mean adding in other ethnicities just to say you did, it means including people who naturally interact with your characters; not deleting them or changing what you would find in reality.
Start small, and be honest with your characters. For example-
Let’s say you are walking down the street and, for the first time, there is a black man walking toward you. Your personal reaction might be slightly uncomfortable merely because of stereotypes. But if the man gives you an acknowledging nod and keeps walking, you would relax and not be so timid the next time.
Say you walk past a few Mexican workers speaking Spanish, and the way they look at you makes you uncomfortable. You can’t help but wonder if they’re talking about you.
You pass a Latina woman at the park with three small children and smile at them, thinking the kids are cute, and that the woman has her hands full.
A black woman drops something and you have to approach her to return it.
As an exercise: Write these small encounters for your characters to become comfortable with the idea of POC (people of color).
Once you start thinking of encountering people in the world, and you realize that they are people too, that certain things will make you uncomfortable, certain things will interest you, other things you won’t think twice about.
Most importantly, people are people. It’s no different than coming from a small family and writing a big family. There are different traditions, foods, clothing choices… but that’s true for any character you write.