Kill a guard with a Broom.
It is impossible to find brooms on the early part of the game. Brooms don’t appear until Rome gets rebuild and quality of life improves. People with brooms usually appear on the later part of the game. So it isn’t advisable to look for them when you are just starting to play the game. But when you do find one, just take the broom from the person who is using it and attack a guard just like you are using a weapon.
If you want to see how this is done, you can watch the video here.