Take some solid fabric and load it. Use one color thread on top, another for the bobbin. When you are done with that piece of fabric, but to save money, change the color of your threads on top and bottom and quilt over what you have already quilted. That way you can tell what you have quilted, and you don't waste the fabric. It will give you a chance to just practice, practice, and practice some more. You may not have a piece to even give to a dog shelter when you are done with all the practicing, but you can get lots of practice in.
Remember to relax and breath. Don't hold the grips tight, just loose with enough control to direct the machine.
Many others have given very good advice. One being, don't expect to be perfect right from the beginning. It does take practice, practice, practice. I still practice in between quilts when I get a chance.
Also draw out patterns on EVERYTHING. Even with your finger in thin air. When using a pencil or pen to draw out a pattern, or practice the movements, don't hold the pencil as you would when you write. Hold it like you would the grips to the longarm. It is a totally different movement.
Good luck and more importantly - have fun!
Phyllis
QuiltingGrannie