General Notes
There are a few basic things to remember when using the paint tools:
First you must set your Preset with the various tools prior to setting your color because when you click on the Preset it will default to a color chosen by PI.Remember to Save often, once you save - clear your undo history.

Painting in Mode versus not

Please note that you may find it much easier to manipulate your painting if you were to paint in object mode.

Advantages to painting in mode is that you can paint a little bit at a time making it an object. If you decide later on down the line that you don't like that particular tree in that spot but it might look better on the other side of the canvas, you can simply move it over.

It also makes it easier should you require to blend certain aspects of your painting.

If you find a setting you like or think you will use often refer to this snapshot tutorial by Debbie Hardisty on how to save your favorite brushes.
Saving Brush Settings to your Easy Palette

Make a folder for your painting textures so if you happen to see a texture you think would work you can crop it out and save it and know where to find it..Then next time you paint just add it to your painting texture panel in PI.

The first one is None and gives a raspy looking appearance

The Split 1 is as the name implies a single stroke

Split 2 is a good choice for some types of bushes ,plants and foilage depending on the transparency used.

Split 3 you have to look close but it is actually split into 3 2 soft outside edges and the center normal

Pepper dot has many uses and size, pressure, color and transparency all give a different look. This can also be used for bushes or trees at a distance or on top of a base to vary the color .You can also use a texture to give it a more varied look .

The Grass setting can be used for so many things besides Grass...It is good for edging a wall for a stone appearance in multicolor
or for making bricks the same way by using the straight line option and adding a texture