Perform the experiment and make observations
Whether you are doing an elementary science fair project, or a high school or middle school science fair project, there are several important things to keep in mind while you perform your experiment.
It is extremely important to make careful and detailed observations and measurements. There are two kinds of observations you can use.
1. Quantitative Observations. This involves measuring variables, which could be size, weight, time, distance, number of things, etc. In the tomato example, there are many variables you could measure: the height of the plants, the number of leaves, the number of tomatoes, the circumference and weight of the tomatoes. Which of these variables are most important to testing your hypothesis?
2. Qualitative Observations. This is describing your observation in words. How did something look, or smell or feel? What happened when you did or changed something. If possible, draw a diagram or take a picture or video to help record and explain what you see. You should also record any problems you have. For the tomato example, this could include a description of which plants are bushier or seem more vigorous, which have greener leaves, which tomatoes are brighter red, whether there were any insect problems, etc.
ALL parts of your science fair experiment MUST be recorded in a notebook or journal of some kind. This includes all the experimental steps you perform and all of your observations. Record the entries in the notebook immediately; do not wait until later, and then have to rely on your memory. For the tomato example, record how many seeds are planted and when, how much fertilizer is given to which plants, when the plants are watered, how many flowers you see, and finally, the measurements of the harvested tomatoes.
There are special, bound laboratory notebooks available. These are helpful but not absolutely necessary. Just make sure that all of the information is recorded all in one place in some kind of notebook.