digitally or the traditional way?
...well digitally, obviously you need an art software. for a beginner, i recommend manga studio debut. it comes with screen tone brushes, panels, balloons, etc for making a comic. the quality may not be as good as something like photoshop though.
if you want to make something highly detailed, invest in photoshop or something like it.
...if traditional, there are a few books on how to do this. i can't really tell ya.