We need some more information before we can help you determine that. What information are you seeking (be as specific as you can) and which part of the National Archives are you planning to visit?
How you prepare for researching with the National Archives depends on your research topic. As Rebecca Collier mentioned, the more specific you are with your information, the better the archivists and research room attendants can be in helping you.
You can check the individual holdings at different NARA facilities, such as Presidential Libraries, to see if they have what you need. Many have online finding aids and reference tabs that can narrow your scope. Facility websites have specific sections on how to prepare to researching at their location. Some will require submitting an application and scheduling a brief interview with the facility archivist.
Call ahead to schedule your research time with the NARA staff. Some locations have regular operating hours and don't require an appointment, but giving advanced notice will allow the staff enough time to gather the materials you'll need. The farther ahead you contact the facility, the better prepared they will be upon your arrival. At the same time, ask what equipment you can bring into the research room. The usage of laptops, cameras, and tablets can vary by facility. Research room staff can also explain research procedures, such as pulling additional holdings and scanning documents.