When mixing your own colours, it helps to have a very basic understanding of colour theory and the colour wheel. The colour wheel is designed to show you which colours are mixed in order to make other colours. The primary colours are those colours from which all other colours are mixed. Orange is a secondary colour, meaning that two primaries (in this case, red and yellow) are combined to make it.
Dab your paintbrush in red paint, so that the tip of the brush is red. You don't need more than a little bit of red. Paint a red circle in the middle of your palette.
Place your paintbrush in yellow paint. You need about 3 times as much yellow as red, so load a significant amount of yellow on your brush.
Mix the yellow paint on your brush with the red circle you painted on your palette. Swirl the colours around. Notice that the colours combine to form a shade of orange.
If your shade of orange is too light, you must add more red paint. If your shade of orange is too dark, you must add more yellow. To create a medium shade of orange, you will always need more yellow than red, because red is a darker colour, and it always requires more of the lighter colour to dilute the darker colours.