if you are in your home directory for example (/home/my_name) and you want to create a directory called my_contacts you would run this command:

mkdir my_contacts

now check to confirm that you craeted a directory called "my_contacts" by running the list command:

ls -F

you will see a directory called my_contacts

* some peole called directories folders. people who called directories "folders" are often used to using microsoft windows and that's why they called directory "Folder"