to go to Science Fair Project Ideas.
"Hypothesis" means "what do you expect to happen in your experiment?" Suppose your research question is, "what happens to seeds if I change the temperatures at which they are kept before they are planted?" The hypothesis might be "the higher the temperature at which seeds are kept, the quicker I expect them to sprout."
It's important to phrase your hypothesis correctly. For example, don't say "higher temperatures are better for seeds." "Better" cannot be measured. Decide on a hypothesis that can be proved in a measurable way. For example, "higher temperatures will make the seeds sprout faster."
It is perfectly fine for your experiment to disprove your hypothesis. If something unexpected happens during your experiment, the project doesn't need to be trashed. You just discovered something new and showed that what we expect is not always what we get.
Do some studying before you decide on your hypothesis. Sources of information include school and public libraries and the Internet. Also, once you have some background, you might consider writing, or e-mailing a scientist who works in the field you've chosen for your project.