It would be preferable if you sign up before you come and also add known family members to your tree. Your time may be limited when you visit and that would get you get you ahead and you could then use your time at the FHL to do further research for your unknown ancestors.
You could, but they can help you easily here. If you want to do it ahead, go to familysearch.org and click get started, then set up your user name and password.
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