there are a couple of ways you could do this, but which way would be the best depends on exactly what you are looking to do.
if you wanted the speed to change during the play back, like speed up, or slow down, then time remapping would be the best way to go.
if all you wanted to do was speed up the 15fps (or 8fps) footage to 30fps, then you could use the interpret footage settings (select footage in project window, then choose file>interpret footage>main) and set the frame rate to 30fps (or whatever you want). now when you take in into a comp it will play back at the frame rate you set ae to interpret the footage at.
if you wanted a little flexibilty, so it was easier adjust the speed, you could just bring the footage into a comp (say a 30fps comp), then choose layer>time>time stretch… enter the percent to speed (or slow) the footage up by.. so if you wanted to go from 15fps to 30fps, enter 200%.
note that you can enter an equation for more difficult speed increases, so for 8fps to 30fps, you could enter 100*8/30 to have ae calculate the speed increase.
Kevin Camp
Senior Designer
KCPQ, KMYQ & KRCW