Bonnie on almost any page on your tree, hover over "tree pages" and select "Tree Settings."
From the resulting page, click on Export, which will start the process of creating a GEDCOM. Depending on the size of your tree, it may take a bit of time.
After that completes, you will see another screen, to download the GEDCOM to your computer. You can then upload it to another site, such as Rootsweb or MyTrees.