By default, you're painting e'erything at once (elevation, rain, temperature, drainage, savagery, volcanism) in the middle of the range-50% of max- one region tile at a time, hard-edged(not that it matters with 1-tile brush)
(Left)clicking on the map will draw- this is not immediately obvious as the entire map is defaulting to the same as your brush.
Right-clicking the map and dragging will move the map around, unless you're painting a pocket world, in which case you already have the whole thing on-screen.
Pressing enter uses the default view, showing biomes. Clicking on ELEV, RAIN, TEMP, DRAIN, SAV, or VOLC will switch to looking at that.
Clicking on Brush = (number) under any of them prompts for a number. For elevation, this is 0-400, others 0-100. This is what you are putting on the map, for that field(elevation, rain, temperature, drainage, savagery, volcanism), when you click.
Clicking
Painting will toggle it between
Painting and
Not Painting- so if you want to only draw one layer at a time, turn them all off but the one type you want to affect.
Clicking
Will be saved toggles it between
Will be saved and
Randomize at gen. This tells it whether to use what you draw or just do its own thing for that field. Don't worry if you click one- it doesn't nuke your data until you leave the world painter, so just change it back.
Clicking the Brush Radius prompts you to enter a new radius for your brush, and clicking Normal Brush toggles between Normal and Smoothing brushes. Smoothing would probably better be thought of as a smooth
ed brush. Normal just sets everything in the area you click to your values, Smoothing sets only the center one and then averages to all surrounding tiles, so making an Elevation 400, radius 5 smoothing brush click generates a nice peak with mountain range, hills, etc.
Pressing space exits the world painter to the detailed list of worldgen settings. Remember (it usually warns you about this) you must press f6 to save the worldgen parameters once you're back at the pick-a-worldgen screen.
For what values result in what, see
http://www.dwarffortresswiki.net/index.php/Advanced_world_generation#World_Painter_Parameters