You are probably wondering why no one has responded. Well, you signed on as a newbie and your project is extremely complex. Do you have any experience with this sort of thing? Do you have a mentor at school? Do you have lots of time and $ for this size project?
I would start with the IR switching circuit. Just get it to trip when the beam is broken. Then work on a way to keep it from responding multiple times as different runners make/break the beam at the start. Next, you will need a similar circuit at the end of the track. They will need to be tied together to measure elapsed time.
Next would be the timer. It will need to be very accurate - probably to 3 or 4 decimal places. Do you have the knowledge/experience for this?
I would think that the tree starting system would be the easiest part of the project. Oh, don't forget that you wanted a false start indicator too.
Perhaps your track club could raise money to buy such an item...