Hey hope you're enjoying Catch the Beat. Sadly when it comes to dashing there's no secret trick to it, it's something you get used to over time. You only really use it when you need to, that is to actually get to the next note in time when just moving there normally won't do. As you play harder maps you'll find yourself using it more frequently but never 100% throughout a map (that I know of). Just keep playing, keep trying and practicing and it'll start to make sense.
As for skins, most people use either circles or squares for their "fruit". You can find a good number of skins from
stalking checking out people's profiles.
ExGon uses squares and
Dusk uses hollow circles, both of which are pretty popular. Myself I use
white circles as I find the hollow one's harder to read. It's really personal preference and just about finding a skin you're comfortable with, wether it's squares, circles or even default. For catcher's check out
this thread, again it's personal preference, just pick something you like.