For a science fair project testing the effects of salt on plants, start with samples that have begun to leaf. Seeds put into salt water may not germinate, but other factors could hinder germination as well. With the plant already leafing, you'll know that it's thriving under your control factors and that any changes are the result of the salinity.
Set out four to six plants and expose each to different concentrations of salt, such as 0.5 gram per liter, 1 g/L, 2 g/L and so on. Tracking the effects and comparing them to the growth of plants not exposed to salinity will not only tell you whether salt has an effect, but how much stronger additional salt will make those effects.
Plants do not register changes overnight unless you're administering a powerful toxin, to get rid of weeds, for example. Treat the plants with salt water for two to four weeks to see meaningful change that you can observe and track.
If you're doing this project in middle or high school, your chemistry department may be able to help you analyze your plants after the end of the observation period for traces of sodium or chlorine. If the plant is suffering adverse effects from saline treatment, you may want to see what residue is in the plant.