You don't layer the top, the quilter will. Some quilters will have a selection of batting, so you may not need to provide your own. The quilter will tell you how large the backing needs to be and if they will accept a vertical seam or even a sheet.
You need to do some research to avoid any disappointments. If possible, get referrals from friends. If you can't do that, ask to see some of the quilter's completed work. Plan on spending some time when you drop the top off. The quilter will probably have ideas, but the more input you can provide the better. If you see a photo of a quilt you like, print it off and bring it with you.
Insist on something in writing. If the quilter doesn't have a formal contract, then at least write something up by hand. At a minimum, it should state the size of the top, type of batting, and thread to be used. The type of quilting should be spelled out: custom, all over, panto etc. If a panto is used, the name of the panto should be stated. The price should be listed. An approximate completion date should be specified. Make sure your name and phone number is clearly listed. You both sign it and each gets a copy. And finally, you and the quilter should look the top over to make sure they don't see any issues.
If you have used a quilter several times, you can probably trust their judgment and don't have to be so formal, but the first time using someone you should be cautious!