First of all, remain calm.
Believe the child, help him/her to express him/herself with their own words, without adding anything and without being overwhelmed by your concerns, your imagination and your knowledge of sexuality. Do not ask him/her to repeat themselves or tell you repeatedly what they experienced.
Quickly accompany him/her to the relevant departments (Police…) to be supported by trained professionals who will gather his/her testimony.
Let the legal professionals question the alleged abuser and conduct their investigations.
Be concerned about the welfare of the child and his/her protection without ever trying to bring about justice yourself.

If you have doubt,s or if the child’s words are not clear, you could take him/her to consult a child therapist.

In any case, tell the child that he/she was right to talk to you, praise him/her for his/her courage and thank him/her for having trusted you.

You can call your country’s child helpline to file a report.