I had a dream about ufo's, i woke up petrified, what does this mean?

No 'dream interpretation' methods can ever be said to be very accurate. (Though I'll admit they often can make a lot of sense!) I'd guess it's mostly just down to that that's what your mind decided you'd be dreaming about that night. As for being fearful the next day, we almost always feel afraid right after awakening from a nightmare, and remembering that could make you feel afraid again later in the day. The fear would highten the more you thought about why you'd be afraid.