Consider the importance of the subject of your study. Perhaps you want to know why old books smell so inviting when you inhale the aroma of the pages. In and of itself, this has little importance, but does it tie into larger questions? Are you studying the question so you can synthesize the aroma for use elsewhere? Will your study answer questions about the biophysical connection between molecular structure of a compound and the emotions the compound stimulates? Select a topic with significance beyond itself.
Research the state of knowledge in the field. For example, to study the pleasant odor of old books, study what is already known about how molecular structure influences the perceived pleasantness of odor. Find out what is already known about the chemical constituents released into the atmosphere when an old book is opened. Your hypothesis would then extend the work of others, without being either obvious or unlikely in the context of what is already known.
Connect the phenomena you want to measure to the parameter it depends on. Science is all about cause and effect. What causes the effect you want to measure? In your preliminary research, for example, you could have found that some of the compounds released from the pages of an old book match the molecular structure perceived as pleasant. So your initial hypothesis could be something like, "Old books smell good because they contain a volatile ester derived from lignin."
Convert your statement into an "if ... then" form. Aim toward specificity and measurability. So your hypothesis about book odors could become something like, "If a book emits more lignin-derived esters, it will smell more pleasant."
Make your statement as clear and measurable as possible. The idea of a hypothesis is to propose an explanation for how some portion of the world works. The hypothesis itself needs to be clear enough that it can be verified or contradicted with testing. The example hypothesis could stand as it is, but you could also modify it to express the heart of your proposition: "If more lignin-derived esters are contained in an air sample, people will perceive the odor as more pleasant."