One thing which is not obvious when growing tomatoes for the first time, is that they are a very vigorous plant. Tomatoes grow very fast, throwing out loads of shoots and they need to be kept in check if they are to produce good fruits. Thinking about it, in April/May the seedling is bit like the ones on the left, and by the end of the summer the plant is over 1.5m tall even having been pruned and stopped back.
It's really important when growing tomatoes to restrict their growth, cut out side shoots and make sure once 3 trusses of fruits have set, you stop off the growing point. This means when you can see 3 distinct tiers of flowers go to the top of the plant and cut off the growing tip. You will need to keep doing this as the plant will just keep growing and growing. It will also grow more and more leaves, which at the end of the season you need to thin down. The plant will continue to produce and abundance of flowers and tiny fruits, which later in the season there will be no time to ripen into fruit so it's best to cut these off too. Growing tomatoes is great, but they need attention every week, not just because they are vigorous but watering and feeding is essential to produce good tasty tomatoes.
The image on the bottom left is of a tomato growing out of a pavement in a main city street which I took last summer. There is a bit of soil in the crevice and I think a discarded part of the sandwich included a tomato and a seed, and there it is growing, and I could not resist taking a photo. Not much chance of tasty fruit but a testament to the vigour of the tomato plant. More about growing tomatoes.