Just found out someone important in my life has lied to me, I'm hurt & a bit upset. What should I do?

Give yourself some time away from them so you have time to cool down, then have a talk with them. You have a good reason to be upset and its important to talk about it and work it out but don't attempt to work things out before you have time to feel. Its important to feel how things affect you because you won't be able to work it out if you do it fresh and angry right after it happened.