There are a couple of ways for approaching tomato growing in a greenhouse, you can either dig and prepare the soil of the greenhouse ready to accept the tomato plants or you can grow the plants in a container such as a plant pot (12″ diameter) or growbag.
Realistically most growbags that are available which can be 35 or 40 litres in size are only enough to support two plants. There are ways in which you can push this out to 3 plants but for the cost of a growbag its probably as well just to stick to 2 plants, particularly if you take good care of them and produce lots of fruit.
If you go for pots then you should steralise them before use, to do this you can use a dilute solution of a suitable disinfectant, something like Jeyes fluid is perfect for this task.
While we are on this subject, you really should wipe down the inside of your greenhouse with disinfectant as well to ensure any residual contaminants that could also cause disease are removed from the glass.