This video on Tiktaalik Roseae gives both a quick rundown on its significance, as well as debunks the main special creationist attacks on its significance:
If you have an hour to spare, this lecture by Neil Shubin, one of the discoverers of this transitional fossil is well worth your time.
Those wanting more detail should consult this paper by Jenny Clack, one of the leading researchers in early tetrapod evolution. The fossil evidence for tetrapod evolution is beyond dispute. All that it needs is for the YEC to have the honesty and humility to read it, rather than arrogantly relying on their own distortion of the Bible, and in the process making an idol of their own ignorance.