You will need to secure your tomatoes to a support (whether that's your greenhouse roof, bamboo stakes, a specialist tomato frame or a homemade structure). Below is a video of Paul, our Gardening Angel, training the tomato stem directly upwards to use the greenhouse roof as the support. You should tie a loop of string around the stem near the bottom of the plant when flowers first appear and then you can gradually twist the plant around the tomato plant stem as the plant grows.