Whether you have bought small tomato plants on line or at the garden centre, or grown from seed, around this time of year April/May the small tomatoe plants will be ready for potting on. The plants in image top left are ready they have filled out the initial seed tray and need a larger growing space, but importantly not too large. If you are not sure check the bottom of the container, can you see the tomatoes roots if so they need more space. Look at the plant if it looks top heavy for the size of it's container it is more than ready to pot on.
It is not adviseable to pot on into the final growing container when the plant is still small, it may seem easier and quicker, but unfortunately the plant will not thrive. This means with Tomatoes it is necessary to pot them on into large pots maybe once, or twice, until putting them a big pot or growbag for the rest of the summer.
Tomato Side Shoots
Looking at the image bottom left, the plant has been potted on into a large pot and if you take a close look at the stem, there are no side shoots. Even when the plant is quite small, it will form side shoots and these need to be nipped out. If left to grow unchecked the plant will put out more and more shoots, growth and leaves at the expense of flowers and fruits. You are aiming to have one upright and to stop the plant sending out side shoots and growing more branches. More information about growing tomatoes and illustration of a side shoot.