At this time of year most of the Tomato plants we have are small and quite young, even so the plant will form sides shoots right from this early stage and these need to be removed. In the first image, this plant is only about 20cms high but if you look closely at the central stem there are small, green shoots running out from the main stem. In the next image these have all been removed.

It is important to remove these shoots all the way through the growing season because Tomatoes are a very vigorous plant and will produce lots of growth, including unnecessary branches which start as side shoots. Unchecked the plant's energy goes into leaf and branch and not fruit.  Left to it's own devices, the side shoots on the plant would be a mass of branches where in fact you are aiming for a single central truck with a measured amount of lateral branches. 

The best way to achieve this is to keep removing the side shoots. You can tell what is useful plant and what are the redundant side shoots as the side shoots always appear at the junction of the stem and branch.

More tips on growing Tomatoes