I'll help ya out! All you have to do is enter "#t=" and then add in the time you would like the video to start at.
Example, if you want a video to start at the 1 minute and 45 second mark you would add #t=1m45s to the end of the Youtube video link. I'll also actually do it so you can see the difference of the links below.
Original Video (Also a good watch!)
http://www.youtube.com/watch?v=XHetrYP8EzI
Same video only now it starts at the 2 minute and 18 second mark.
http://www.youtube.com/watch?v=XHetrYP8EzI#t=2m18s
Hope this helps!