One of the questions that comes up time and time again is how do you prune tomatoes and which suckers should you remove and leave. It is very tricky to answer these questions in written text. So the video that you can watch below is a pretty comprehensive insight into how you remove suckers and when to leave suckers in place. You should note from the video that typically you do not remove suckers from a determinate type plant but it is generally a good idea to remove the suckers from an indeterminate type plant.
Using a string system as shown in this video is OK for a greenhouse, but not so convenient outdoors. My preferred method is to use a good strong stake and tie the plant off using soft string just below the fruit truss to take the weight. So as the plant grows and you see the blossom appear you simply secure the main or secondary stem to the stake a little way below the point the blossom or fruit truss has appeared. Also when you do this take a look around the plant to see if there are any new suckers coming between a leaf stem and the main stem below the blossom, because you may need to remove these.
One of the main reasons for pruning tomatoes is to increase yield. This is achieved by ensuring that the fruit bearing trusses get all the goodness and it is not wasted on growth that can potentially block sunshine and will probably not mature quickly enough to produce a crop of tomatoes. The tomato season, outdoors at least, is restricted to the time summer sunshine is available.