Tomatoes are really vigorous, and I include the image left to show just how determined growers they are. This is a tomato plant growing in a city centre in the scant soil provided by a pavement crack. The original seed probably came from a sandwich. This plant will produce lots of leaves but sadly few tomatoes.
If we Tomatoes unchecked, not only will they make lots of side shoots, they will also grow up and up. The plant will produce masses of growth and leaf at the expense of the flowers and fruit, so to get a good crop, it is necessary to constrain the plant.
Later in the season, around August / September, it is necessary to "stop off" the tomato plants. This means pinching out the growing tips at the top of the plant and stop the plant growing up any further. Stop off at the growth tips at the top of the plant when there are 3-4 (outside grown) and 4-5 (greenhouse grown) trusses, which are layers of flowers. The exact timing of when to stop the plant growing depends on when it has set enough trusses, which depends on growing conditions.
Later in the season it is a good idea to thin out the tomato foliage which diverts the plant's energy into making fruits and not more and more leaves. Thin out foliage as soon as the flowers appear and carry on as the plant fruits. Towards the end of the season, the plant shouldn't have masses of leaves on it. Be bold, cut them off. In the peak of the growing season, I remove lots of leaves each week when checking the tomatoes. This also aids ventilation, which helps to reduce the risk of diseases.
It is time-consuming but essential to take off some leaves and thin growth regularly. Continue taking out the growing points as the plant will strive to keep growing. This means once you have cut off the top of the plant to stop it growing, you will need to do this again, and again, as it will keep trying to grow through the greenhouse ceiling.
Cordon tomatoes, the upright varieties grow so tall and need sturdy supports. There are commercial metal supports you can buy, but simple bamboo canes and wooden stakes are just as good. If the fruit is heavy or large you may need to support it with extra ties, and again raffia is very good for this.