how can i join the dark brotherhood

You must kill someone who didn't attack you first (or would if they could see you), there is lots of NPCs to choose from but most will give you bounties but there is a few which won't specifically if you have an NPC following you for a quest who isn't essentical then quite often you can kill them without losing any reward, guides give you a good list, or you can just try to kill someone and not get caught. Then when you next sleep you will recieve a visit and an invitation

thank you ill try it