For your first song, I’d recommend choosing a short and repetitive song and something that is not too dense. It should have a simple and distinctive 4/4 beat and not change throughout the song. You may want to choose a song with a definitive ending so you don’t have to concern yourself with fade outs. Before doing any charting, make sure the tempo map is nearly perfect since that will make everything else far easier. As far as which instrument to chart first, I’d suggest charting the one that you like to play the most. Try to chart all difficulties. Start with Expert and work your way to easy. It only takes a couple extra hours to do all difficulties per instrument once you get the hang of it. You will make mistakes so assume that your first song will be a throw away.