How do I know which is the null hypothesis, and which is the alternative hypothesis?

Generally, the null hypothesis is the one we want to reject, and the alternative is the one we want to prove is true. For example, if we were testing whether growing in the Sun made a plant grow healthier, we could set H0(null) to be that there is no difference in health with or without Sunlight, and H1(alternative) to be that Sunlight makes the plant grow healthier.