The two coloured squares on the toolbar show you your current 'foreground' and 'background' colours.
Click on one of these. The colour picker appears.
Select the colour you want by moving the pointer up and down the coloured bar. Select the shade you want by moving the spot over the shaded square. Click OK.
If you want to match a colour which is already in your picture, simply move the cursor over your picture and click there.
Click on the curved arrow to switch foreground and background colours.
Select the brush tool from the toolbar (2/3 down on right). If the pencil tool appears there you will need to hold the button down for a moment until the option to select brush appears.
Select the size and type from the 'Brush' drop down near the top left corner of the screen. Start with a hard round, diameter, say, 45. Set 'opacity' to 100. Now try painting with this brush on your picture. Note that it uses the foreground colour you selected above. Try the effect of different size brushes.
Now select a soft round brush. Try using this. Note the soft edges.
Now set the opacity slider to, say, 20. Note how you can see through the painting. Note how the colour builds up as you go over the same area several times.
You will use brushes in several different ways in editing your images, not only for 'painting'.
With photographs you will almost always use a soft brush with low opacity. Hard brushes will look unreal. Low opacity gives you much more control.
e.g. Painting a shadow
Ordinary painting is not often used on photographs. One example where it is useful is to produce a shadow. We will learn later how to copy part of one picture onto another. For now look at the first attempt here and how it looks obviously stuck on. The second picture is the result of painting a shadow on the first with a soft round brush, black paint and opacity around 12%.