The first you need is a pen to draw with. At a time Wacom was the only choice, but now you can use various other brands or use a Surface PC.
I can recommend giving
PAP 4.0 Pro a try - it is a free download, and has loads of features - adjustable onion skin and pressure, blue pen for sketching, and a really nice Zoom Menu to quickly zoom, drag and rotate the canvas.
_Slaugh_ recommended Pencil2D, which is quite good too. Nice and simple. I kinda think of it as MS Paint, but with animation in it.