As a fellow native English speaker learning Polish, I think of it like this:
używać - to use (imperfective - the process is going on)
użyć - to use (perfective - the process is done)
When conjugating, I generally think of it like this:
Past:
używałem - I used/was using (habitual, indefinite)
użyłem - I used (done, finite, no longer using)
Present:
używam - I am using/I use (habitually, generally)
Future:
będę używał - I will be using/will use (extended, undetermined period)
użyję - I will use ('get used', finite, will use maybe once and stop using)
Since Polish has fewer tenses than English, it can be tough to translate exactly! E.g. użyłem can be both "I used", "I had used", "I have used" etc., używałem can be "I used", "I was using", "I had been using", "I have been using" etc.
Another useful way of thinking of it is when you combine perfect and imperfect in a sentence. Like this: "przeczytałem książkę podczas gdy oglądałem telewizję", I guess it would be translated as "I finished reading the book while I was watching TV", the first verb being perfective and the second imperfective.
I hope this offers some clarity! Obviously, what Andres said is also very useful!
Nick