How do you get into the darkbrotherhood and where is the thieving guild..

1) To get in the Dark brotherhood, you must have commited a Murder (I got in for murdering about 5 people!). Afterwards, you must slep in a bed, a bloke in a black robe comes to see you. There's your entry ticket.
2) The Thieves guild is located in the Imperial City Waterfront. It's qite hard to find, so talk to some of the beggars there.

Hope this answers your questions!

The Dark Brotherhhod is available after murdering some people (Especially guards) It will say "your actions have been observed by an unseen force" in the top left corner. Then sleep in a safe bed, and Lucien Lachance will visit you, and give you a mission.

The thieves guild becomes available after you serve time in an imperial prison. A dark elf chick will give you a note "from a friend". Go to the shacks at the waterfront anytime between midnight and 2am, and you should see Armand Christophe holding a torch and standing with a wood elf chick and an argonian behind a small wall at the end which is away from the water

To get into the thives guild you need to have spent time in prison

Dark brotherhood--- kill an innocent citizen. In the top left corner it will say "your killing was observed by unknown forces...".

Thieves guild(first way)---look at a grey fox wanted poster and talk to a begger. Get their disposition at 70 or above and talk about the grey fox. He'll ask you if your looking for him. Say yes and he tells you to go to the garden of dareloth at midnight. Talk to armand christophe and get his disposition to 70 or above. Talk about joining the thieves guild and he'll let you try for the guild.


Thieves guild (second way)---hit a guard in the imperial city and spend time in jail. After you leave prison, try waiting for a while. It will stop it when a dark elf will give you a note. She will say "take this. It's from a friend".