From a first timer opinion, the advice that I would look back and give myself is find a song that has:
- a clear 4/4 time signature so tempo mapping isn't crazy
- a drum beat from the beginning (not one that has none or a drum beat that comes in at a later time)
- instruments you can CLEARLY hear differentiation between (not heavily distorted mixes or so fast you lose everything trying to chart one thing)
- a midi file attached to the song (to give you at least SOME basis to work from)
- vocals that don't have a lot of runs, just straight notes sung long
- a length of three and a half minutes or shorter
I think that would have made it less challenging for me.