Replying to Filming at night

don't really know a whole lot about filming at night. I got a t3i and filmed some urban in the past but iv always resorted to bumping up my iso which results in really poor hd and grainy shots. stept always films at night and the shit they film looks crystal clear. anyone got any tips or know how to get 1080 shots at night in the dark. planning on going down to boston in a little bit and shooting around at night with some friends so help would be appreciated