就 (like many other Chinese characters) has more than one meaning. So it can be used in many different ways.
When you use 一 and 就 together, it means "as soon as.." for example
我一来他就走了 - as soon as I came, he left.
就 can also be used to show that something happens or has happened early:
我下午三点就到机场了 - I arrived at the airport [as early] as 3pm.
note that when you use "就" in this sense, 了 must also be used (to show the completion)
"就“ can also mean "only/just". For example if someone asks you "要喝几杯?" (how many cups would you like?) you can answer 就一杯(just one).
There are also other ways 就 can be used! Make sure you pay attention to how it is being used ! :)