This "social darts" club in Old Street is super fun and super straightforward. The setup is more like bowling than anything else (ie the scores are computerised, and the rules come up on a computer screen), and the staff are very helpful if you're a newbie.
The other bonus is that they have a smashing non-alcoholic menu. There's the usual soft drinks, but also interesting offerings from Fentiman's, Bottlegreen, and Schweppes.
Find out more here.