CONGRATS!!!! Don't be scared. By definition, you will be cool... at least until he or she is in high school. Even then, I'm sure you'll still be the young cool hip aunt.
Based on my niece and nephew (5 and 7), all it takes to be the cool aunt is giving them your time. Read a story. Paint a picture. Play a game. Laugh at their attempt to tell a knock-knock joke..
I admit, my Princess Peanut and My Liege are spoiled. I have a driver's license and live only 15 minutes away and I see them almost every week or two.