Impulze:

Click the button in the top right that looks like a folder with slider bars on it. It opens up the brush palette. You can also go to Window -> Brushes or press F5

Now go to "Scattering" and play with those settings. It makes your brush appear in random locations instead of a smooth, straight line. You can also make it stronger or weaker at random, different sizes at random, and a number of those modifications.