First, I'd wonder if I was still dreaming. 
I'd probably spend the first hour trying to determine if I was insane, once I'd figured out that I wasn't dreaming. 
Then I'd probably be really excited. 
I'd get the hang of having a super long tail (which I'd be extremely excited about!). Then I'd probably stare at myself in the mirror for a while until I felt pretty used to how I looked. Then I'd go out and see if I could tell my friends about it without them freaking out. 

' And then I'd probably tell my sister. And maybe the rest of my family, if they could handle it. 
And then I'd just be super happy.