If you are painting with acrylics it does not really matter where you start on the canvas, because they dry pretty quickly and there is little risk of smearing with your hand. Also, it's recommended that you use long brushes and hold them far from the bristles, which has several benefits including no smearing.
Some people start from the focal point, others start from the background, I like to work on everything, keeping the same level of progress all over the canvas. You'll figure out what works for you after painting several pieces.
To gauge proportions, you may draw a grid on the reference photo and a proportional grid on the painting surface, that way you can look at each rectangle and compare how the different lines intersect for the correct placement of elements.