Want to Be a Better Ally? Believe Women
Here's something immediate and profound cis men can do to be better allies and better feminists:
Believe women.
Believe that women are the most reliable sources of information about their own experiences. Don't believe that they're misinterpreting something, reacting hysterically or emotionally, or that if it happened to you, you would handle it better.
Believe them when they say sexism is everywhere, even if you don't see it.
Believe them when they tell you a doctor didn't listen, or refused to treat them, or abused them, or incorrectly treated them.
Believe them about their experiences with workplace harassment and pay inequity.
Believe them about bullying at school and out in the world.
Believe them when they share a negative experience with someone you like. Don't tell them, "Well, I've always had good experiences with him." Expect that women's experiences with your favorite doctor, best friend, closest family members, real estate broker, insurance agent, and most beloved boss will all be different from your experiences. The very nature of oppression is that oppressors treat some people differently from others.
This simple rule extends well beyond women's experiences. Any person who is privileged relative to another can become a better ally by listening and believing.
Believe trans people, and Black people, and poor people, and disabled people, and immigrants. Believe that you can never know more about someone else's experience than they do. Thinking otherwise is a prerequisite to oppression.