This topic is well explained under "Exporting Images". It's as simple as deciding whether you'd like to export the modified version that you've been working on or your original untouched master under the File>Export menu. Once that Export Dialogue opens you can navigate through your directory (above all the pop-up menus) to where you'd like that file placed (Desktop, Pictures Folder etc.)Choose the format type and size that you'd like that image to be in under the export presets pop-up menu. You'll have more options for your export if you choose Edit from the preset drop down menu. There you can even assign your particular Lab's printing profile if you've loaded that into your computer under Library/ColorSync/Profiles, if you've done that correctly than it should show up in your list of ColorSync Profiles so your image will be exported with that profile built into it. You can also create subfolders for that exported file to live in and rename the file that you are exporting within the export dialogue box as well.
So yes Aperture will create a new file outside of Aperture wherever you decided to put it, to the exact specs that you specify within that export dialogue box.
Using your lab's website upload page, depending on how it's set-up, you'll need to navigate to where you're exported file is sitting through their upload program and then you should be good to go.
