I just bought one too, and I had the same question. In researching books that might help me learn how to learn to use the longarm, I came across the following quote in Machine Quilting Solutions by Christine Maraccini:
"The general rule is that it takes one full year to become comfortable and proficient on a longarm."
Daunting, isn't it? Actually that estimate makes me feel better about practicing. I'm not expected to get up to speed in the first day, week, or month. I have a whole year!
I've been practicing all kinds of different quilting patterns on a D9P that I made just for practicing. When it's done it will have a little bit of everything, so it will look pretty strange. I'm also going through this site and trying out the fillers that look interesting:
http://www.daystyledesigns.com/365project.htm
I am drooling over quilts that Shelley and Ronda K Beyer have posted recently. I don't expect to reach that level even after the first year. Maybe not even after 100 years.
What kind of machine did you get, and how are you practicing now?