When I got my longarm I found two, old, worn sheets that I used to sandwich scraps of batting that I had sewn together.
I then stitched lines top to bottom, about 12" apart as well as both sides and top and bottom. This became my my 'practice quilt'. It was easy to attach to the frame since I wasn't dealing with 3 free layers.
On ebay I found a cone of water soluble thread for a good price. This went in to my bobbin. Top thread was an old cone that I had picked up at a garage sale - too questionable for an actual quilt but perfect for practice. Then it was off to the races! I was able to practice for muscle memory, control of the machine, etc.. Once the quilt was filled, into the washer it went and when done, the top thread just pulled off.
I still use this for trying out new designs although I am seeing alot of needle punctures now so it probably doesn't have a lot of life yet. But it was cheap, easy to make and gave me the practice and confidence I needed. Have fun with your new machine!