A hypothesis is an educated guess, so what I think would be a good way to make a hypothesis is to research the topic a little bit, and make an assumption based on what you learned.
Like, say I'm interested in gases in balloons. I researched, and now I know that helium-filled balloons rise to the ceiling and balloons filled with regular air don't. I'm going to say, based on that, that helium is less dense than air which causes it to rise to the ceiling. I may or may not be right, but it doesn't matter right now. All that matters is that I can come up with an experiment to test my hypothesis! So my experiment would be testing if helium is actually less dense than air.
