Tau are one of the easier armies to paint, Heretian on here has some nice Tau that he has airbrushed.
There are loads of good tutorials on Youtube, also when you get it spend time spraying paper or cardboard, to get an idea of the paint coverage at different PSI how much you pull back on the widget on the brush.
Ideally you want a side or gravity airbrush and a two action one, but that is where you pull back on the widget for air, then press down for paint, so gives you more control over a single action which is paint and air at the same time