Summarize this article in one sentence.
When your tree is about 2 years old and 1 metre (3.3ft) tall, with 3 or 4 strong lateral branches, you can begin the first stages of shaping. Your tree will not be bearing any fruit until it is about 3 or 4 years old, so this pruning is just to start getting it formed into a shape that will be conducive to healthy growth and fruit bearing. When your tree does start bearing fruit, it will be in the fall, so the ideal time to prune is right before it's about to begin its new growth for the year. Try to prune your tree on a dry day so that the new cuts don't get too wet and vulnerable to infection. Olive trees grow slowly and overall don't need a lot of pruning. Pruning once per year is plenty. It's important to maintain a balance between shaping your tree into a form that will be its main structure for life and allowing your tree to grow and build energy reserves. Too much pruning will stunt a young olive tree's growth. If your tree is a few years old and still isn't 1 metre (3.3ft) tall with a trunk and more than 3 or 4 strong lateral branches, you can hold off on pruning for another year. The healthiest olive tree shape is that of a wide martini glass, with the trunk being the glass stem. Most branches should grow laterally and slightly up. The center of the "glass" remains light in branch density to allow light into the center of the tree. To begin achieving a martini glass shape, choose 3 or 4 strong branches that are growing mostly out and slightly up from the trunk to keep as the main structure for your tree. Leave smaller branches coming off these branches, even if they are growing down. You can cut other small, weak, or vertical branches aside from these main 3 or 4. If your tree only has 2 strong lateral branches, you can prune others that seem very weak or too vertical, but next year you'll want to look for another 2 strong branches to keep. Eventually you want your tree to have 4 strong lateral branches for its main structure.

Summary:
Begin shaping when your tree is 1 metre (3.3ft) tall. Prune once a year in the late spring or early summer. Avoid cutting too much on young trees. Aim for a martini glass shape. Choose 3 or 4 strong lateral branches to form your main shape.