You can drag the footage to the left, until the point you want is aligned with the start, or you can type in an exact number in the In column in the timeline window. By default this column is not shown, to show it right-click in the column headings, where it says Source name … Parent & Link and select In from the list.
You can click the number and enter
-24. in the text field. **Pro tip:**you don't have to type the zeroes, AE will add them for you if you type
., this works for hours and minutes too, so if you wanted 1:02:00 you could just type
This brings me to the third way to do it. In the current time box, type
-24. to set the current time to -24, then hit [ to set the layer's in point to the current time (the comp window will be grey because you're outside the composition). Hit home to go back to the start of the comp.