Take some fabric out of your stash. Or, if you can, go to JoAnn's and get LOTS of cheap muslin. Put that on the frame, with batting and quilt away. Just do whatever comes to mind. Now, remember, you won't do it the way you see in your mind. It takes practice. Start with one design. Say, a simple meander, practice that until you feel comfortable, then move on to something else. I quilted many of my first quilts on my frame with just a simple meander.
Just relax, and remember to breathe!
Just one more quick thing - if you think what you are quilting doesn't look good - write your name. You will be amazed at how good that looks. Then, think of how many times you have written your name. That proves that practice is the key.
Have fun with it.