You must be able to explain the variable changes and measure it. Then you run the experiment without these changes. This is called the control experiment. This allows measurement of change. When you do experiments, record all measurements made.
Data can be amounts of chemicals used, how long something is.
Qualitative data is also useful and should be recorded. For example, it smelled bad, the color changed, it got moldy. Analysis Put your data into graphs and tables. Are there patterns, Do statistics if you know how. This will help you understand your experiment and produce a conclusion.