Cheap places to stay in London?

Hello there :) So, I’ve been trying to plan a trip to London and I’d really like some advice on where to stay. I’m looking for the cheapest place I can find (I know anything in London must be very expensive..). Let’s say the most I can pay is 100 euros per night. Um… I’ve already found cheap airplane tickets but I really really have no idea where to stay. Also, if you happen to know any nice cheap place outside London but close enough, let me know.
Any other travelling advice will be soooo appreciated, I’m 18 and I’ve never been anywhere outside the country (I’m from Greece).

Oh! Aaaand if you know any nice places to visit (except the obvious ones), let me know!