A hypothesis is an unproven explanation for the observed phenomena. In its simplest form, a hypothesis is an "educated guess" or intuitive hunch that is proposed as a possible answer to the question you're interested in answering. There's a couple of things to know about hypothesis building before you get started:
A hypothesis is not a question, it is a statement
For example, "over a given time period, plants will grow taller at higher temperatures" is a hypothesis, whereas "over a given time period, will plants grow taller at a higher temperature?" is a question. They're generally related, but they're not the same.
A hypothesis must be testable
The hypothesis does not need to be "correct" (after all, there's really no way to know that at this point) but you do have to be able to test whether it is correct or not. In our example from above, we can test the hypothesis by growing the plants at different temperatures, and measuring their heights after a set amount of time. Thus, we have a way to measure the effect of interest and test our hypothesis.
A hypothesis comes before the experiment, not the other way around
We call this an a priori hypothesis, meaning that we made the hypothesis before we ran the experiment and learned the answer. Sometimes, because we want to show that we knew what we were doing, we feel the need to change the hypothesis we started with, so that it better reflects the results we got. This is known as HARKing (Hypothesizing After Results are Known), and is not considered to be good science. It's important to present your hypothesis as you originally developed it, and then discuss what you have learned about the topic based on your testing of that hypothesis.