It is important to understand the topic you are dealing with when asked to put forth a hypothesis and design an experiment around something. Acid rain tends to be precipitation that has combined with factory emission sulfate particles in the air and produced a rain that is lower on the pH scale than "regular" rain, which is close to around 7 pH, right in the middle. Acid rain tends to be bad for plant growth, and has harmful consequences for fish and wildlife in general.
Knowing that acid rain would not tend to promote a health plant growth, we could next put forth the hypothesis (an educated guess). A logical hypothesis at this point would be something like "Plants that receive acid rain will show less growth than plants that received regular rain." Another hypothesis could also be employed, if you wanted to emphasize the effects acid rain has on wildlife, such as "Fish that live in an acid rain environment live shorter lives than fish living in regular water environments."
The next thing you would need is an experiment designed to show the difference acid rain would make in either of the two scenarios depicted in the two hypotheses above. In the case of plants, three experimental plants would be watered, plant 1 would receive strong solution of acid rain water, plant 2 would receive weak solution of acid rain water, and plant 3 would receive regular rain water. You would also need 3 more plants to serve as a control group, that received regular water for all three. Over a reasonable period of time, you would ater the plants and measure their growth. The results (data) would either support your hypothesis or disprove your hypothsis.